Filed Pursuant to Rule 424(b)(2)
Registration No. 333-224357
Registration No. 333-224357-01

The information in this prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been filed with the Securities and Exchange Commission. This prospectus supplement is not an offer to sell these securities and is not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED APRIL 20, 2018

PROSPECTUS SUPPLEMENT TO PROSPECTUS DATED APRIL 20, 2018

CNOOC FINANCE (2015) U.S.A. LLC

(formed in the State of Delaware)

US$                         % Guaranteed Notes due 20            

unconditionally and irrevocably guaranteed by

CNOOC Limited

(incorporated with limited liability in Hong Kong)

The             % Guaranteed Notes due 20            (the “20            Notes”) will be issued in initial aggregate principal amounts of US$            by CNOOC Finance (2015) U.S.A. LLC (the “Issuer”). The 20            Notes are referred to in this prospectus supplement as the “Notes.” The Notes will be the direct, unconditional, unsubordinated and unsecured obligations of the Issuer, unconditionally and irrevocably guaranteed by CNOOC Limited (the “Company”). We refer to the guarantees by the Company as the “Guarantees.”

The 20            Notes will bear interest from             , 2018 at the rate set forth above, payable semi-annually in arrears on             and             of each year, commencing             , 2018.

The Issuer may redeem the Notes at any time upon the occurrence of certain tax events. At any time, the Company or the Issuer may, at the Company’s or the Issuer’s option, redeem the Notes, in whole or in part, at a redemption price equal to 100% of the principal amount of the Notes redeemed plus the applicable premium as of, and accrued and unpaid interest, if any, to the redemption date. For a more detailed description of the Notes and the Guarantees, see “Description of the Notes and Guarantees” in this prospectus supplement and “Description of Debt Securities and Guarantees” in the accompanying prospectus.

Investing in the Notes involves risks. See “Risk Factors” beginning on page S-10.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the Notes or the Guarantees or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Note:

Application will be made to The Stock Exchange of Hong Kong Limited (SEHK) for the listing of the 20             Notes by way of debt issues to professional investors (as defined in Chapter 37 of the Rules Governing the Listing of Securities on The Stock Exchange of Hong Kong Limited and in the Securities and Futures Ordinance (Cap. 571) of Hong Kong) (together, “Professional Investors”) only. This document is for distribution to Professional Investors only. Investors should not purchase the 20             Notes in the primary or secondary markets unless they are Professional Investors and understand the risks involved. The 20             Notes are only suitable for Professional Investors.

SEHK has not reviewed the contents of this document, other than to ensure that the prescribed form disclaimer and responsibility statements, and a statement limiting distribution of this document to Professional Investors only have been reproduced in this document. Listing of the 20             Notes on SEHK is not to be taken as an indication of the commercial merits or credit quality of the 20             Notes or the issuer and guarantor, or quality of disclosure in this document. Hong Kong Exchanges and Clearing Limited and SEHK take no responsibility for the contents of this document, make no representation as to its accuracy or completeness and expressly disclaim any liability whatsoever for any loss however arising from or in reliance upon the whole or any part of the contents of this document.

We expect to deliver the Notes to investors through the book-entry delivery system of The Depository Trust Company and its direct participants, including Euroclear Bank SA/NV and Clearstream Banking S.A. on or about             , 2018, which is the fifth business day following the date of this prospectus supplement. Purchasers of the Notes should note that trading of the Notes may be affected by this settlement date. See “Underwriting (Conflicts of Interest).”

Joint Global Coordinators, Joint Lead Managers and Joint Bookrunners

(in alphabetical order)

Joint Bookrunners

(in alphabetical order)

The date of this prospectus supplement is             , 2018

Notice to Prospective Investors in Hong Kong

You are advised to exercise caution in relation to the offer. If you are in any doubt about any of the contents of this document, you should obtain independent professional advice. The Notes are only available in Hong Kong or to persons resident in Hong Kong who are (a) “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) acquiring the Notes in circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding up and Miscellaneous Provisions) Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance. Each purchaser of the Notes in the United States who is a resident of Hong Kong, by accepting delivery of this prospectus supplement and the accompanying prospectus, will be deemed to have represented, agreed and acknowledged that (a) it is a “professional investor” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) it is acquiring the Notes in circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding up and Miscellaneous Provisions) Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance.

No advertisement, invitation or document relating to the Notes, which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) has been issued or had in possession for the purposes of issue or will be issued or had in possession for the purposes of issue, whether in Hong Kong or elsewhere, other than with respect to Notes which are or are intended to be disposed of only to persons outside of Hong Kong or only to “professional investors” as defined in the Securities and Futures Ordinance and any rules made under that Ordinance.

PRIIPs Regulation / Prohibition of Sales to EEA Retail Investors

The Notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the European Economic Area (“EEA”). For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client as defined in point (11) of Article 4(1) of MiFID II (as defined herein); (ii) a customer within the meaning of Directive 2002/92/EC, where that customer would not qualify as a professional client as defined in point (10) of Article 4(1) of MiFID II. Consequently no key information document required by Regulation (EU) No 1286/2014 (the “PRIIPs Regulation”) for offering or selling the Notes or otherwise making them available to retail investors in the EEA has been prepared and therefore offering or selling the Notes or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPS Regulation.

TABLE OF CONTENTS

Prospectus

We are responsible for the information contained and incorporated by reference in this prospectus supplement, the accompanying prospectus and in any related free writing prospectus we prepare or authorize. We have not authorized anyone to give you any other information, and we take no responsibility for any other information that others may give you. If you are in a jurisdiction where offers to sell, or solicitations of offers to purchase, the Notes offered by this document are unlawful, or if you are a person to whom it is unlawful to direct these types of activities, then the offer presented in this document does not extend to you. The information contained in this document speaks only as of the date of this document, unless the information specifically indicates that another date applies.

This document includes particulars given in compliance with the Rules Governing the Listing of Securities on The Stock Exchange of Hong Kong Limited for the purpose of giving information with regard to the Issuer and the Guarantor. The Issuer and the Guarantor accept full responsibility for the accuracy of the information contained in this document and confirms, having made all reasonable enquiries, that to the best of its knowledge and belief there are no other facts the omission of which would make any statement herein misleading.

S-i

ABOUT THIS PROSPECTUS SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS

This document is in two parts. The first is this prospectus supplement, which describes the specific terms of this offering. This prospectus supplement also incorporates by reference the information described under “Where You Can Find More Information About Us.” The second part is the accompanying prospectus dated April 20, 2018. The accompanying prospectus contains a description of our debt securities and gives more general information, some of which may not apply to this offering.

If the description of this offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information in this prospectus supplement.

This prospectus supplement, the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus include particulars given in compliance with the Rules Governing the Listing of Securities on HKSE for the purpose of giving information with regard to us. We accept full responsibility for the accuracy of the information contained in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus and confirm, having made all reasonable enquiries, that to the best of our knowledge and belief there are no other facts the omission of which would make any statement herein misleading.

In this prospectus supplement, unless otherwise indicated, references to “we,” “us,” “our” and the “Company” refer to CNOOC Limited, or CNOOC Limited and its subsidiaries, including CNOOC Finance (2015) U.S.A. LLC (the “Issuer”), as the context requires. References to “CNOOC” are to China National Offshore Oil Corporation and its subsidiaries (other than CNOOC Limited and its subsidiaries). References to “China” and the “PRC” refer to the People’s Republic of China and, solely for the purpose of this prospectus supplement, exclude the Hong Kong Special Administrative Region, the Macau Special Administrative Region and Taiwan.

References to “Renminbi” and “RMB” are to the legal currency of China, references to “U.S. dollars” and “US$” are to the legal currency of the United States, and references to “Hong Kong dollars” and “HK$” are to the legal currency of the Hong Kong Special Administrative Region.

There has been no material adverse change in the financial or trading position of the Company since December 31, 2017, except as set out in the “Risk Factors” and “Certain Financial Data” sections of this prospectus supplement.

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FORWARD-LOOKING STATEMENTS

This prospectus supplement and the documents incorporated by reference include “forward-looking statements” within the meaning of the United States Private Securities Litigation Reform Act of 1995, including statements regarding expected future events, business prospects or financial results. The words “expect,” “anticipate,” “continue,” “estimate,” “objective,” “ongoing,” “may,” “will,” “project,” “should,” “believe,” “plans,” “intends” and similar expressions are intended to identify such forward-looking statements.

These forward-looking statements address, among others, such issues as:

These statements are based on assumptions and analysis made by us in light of our experience and our perception of historical trends, current conditions and expected future developments, as well as other factors we believe are appropriate under the circumstances. However, whether actual results and developments will meet our expectations and predictions depend on a number of risks and uncertainties which could cause our actual results, performance and financial condition to differ materially from our expectations, including those associated with fluctuations in crude oil and natural gas prices, our exploration or development activities, our capital expenditure requirements, our business strategy, whether the transactions entered into by us can complete on schedule pursuant to their terms and timetable or at all, the highly competitive nature of the oil and natural gas industry, our foreign operations, environmental liabilities and compliance requirements, and economic and political conditions in the PRC and overseas. For a description of these and other risks and uncertainties, see “Risk Factors” and other cautionary statements appearing in this prospectus supplement and the documents incorporated by reference.

Consequently, all of the forward-looking statements made in this prospectus supplement and the documents incorporated by reference are qualified by these cautionary statements. We cannot assure you that the results or developments anticipated by us will be realized or, even if substantially realized, that they will have the expected effect on us, our business or our operations.

S-iii

S-1

S-2

S-3

S-4

S-5

CERTAIN FINANCIAL DATA

Recent Developments

We achieved a total net production of 120.1 million barrels of oil equivalent (“BOE”) for the first quarter of 2018, representing an increase of 0.8% year-over-year.

For the first quarter of 2018, we made six new discoveries and drilled fifteen successful appraisal wells in offshore China.

Our unaudited oil and gas sales revenue reached approximately RMB42.54 billion for the first quarter of 2018, representing an increase of 10.8% year-over-year, mainly due to the significant increase in international oil prices. During the period, our average realized oil price increased by 23.0% year-over-year to US$63.50 per barrel, which is in line with the international oil prices trend. Our average realized gas price was US$6.47 per thousand cubic feet, representing an increase of 7.8% year-over-year, mainly due to the increased gas production of high price gas fields.

For the first quarter of 2018, our capital expenditure for exploration, development and production reached approximately RMB9.66 billion.

S-6

The following table sets forth our net production data for the periods indicated:

Note:

S-7

The following table sets forth our unaudited revenue and capital expenditure for the periods indicated:

Note:

Non-GAAP Financial Measures

We use adjusted EBITDA to provide additional information regarding our operating performance. Adjusted EBITDA refers to our profit from operating activities before the following items: (i) depreciation, depletion and amortization and (ii) impairment and provision.

Adjusted EBITDA is not a standard measure under International Financial Reporting Standards (“IFRS”), Hong Kong Financial Reporting Standards (“HKFRS”) or generally accepted accounting principles in the United States (“U.S. GAAP”). As the oil and gas industry is capital intensive, capital expenditure requirements may have a significant impact on the operating revenues of companies with similar operating results. Therefore, we believe the investor community commonly uses this type of financial measure to assess the operating performance of companies in our market sector.

As a measure of our operating performance, we believe that the most directly comparable IFRS and HKFRS measure to adjusted EBITDA is profit from operating activities. We operate in a capital-intensive industry. We use adjusted EBITDA in addition to profit from operating activities because profit from operating activities includes certain accounting items associated with capital expenditures, such as depreciation, depletion and amortization and impairment and provision. These accounting items may vary between companies depending on the method of accounting adopted by the company. By minimizing differences in capital expenditures and the associated depreciation, amortization and impairment expenses, adjusted EBITDA provides further information regarding our operating performance and an additional measure for comparing our operating performance with other companies’ results. Funds depicted by this measure may not be available for debt service due to capital expenditure requirements and other commitments.

S-8

The following table reconciles our profit from operating activities under IFRS and HKFRS to our definition of adjusted EBITDA for the periods indicated:

Note:

You should not consider our definition of adjusted EBITDA in isolation or construe it as an alternative to profit from operating activities or as an indicator of operating performance or any other standard measure under IFRS or HKFRS. Our definition of adjusted EBITDA does not account for depreciation, depletion and amortization and impairment and provision. Our adjusted EBITDA measure as described above may not be comparable to similarly titled measures used by other companies.

S-9

RISK FACTORS

In considering whether to purchase the Notes, you should carefully consider the risks described below in this prospectus supplement, in the accompanying prospectus and in the documents incorporated by reference in this prospectus supplement and the accompanying prospectus. If any of these risks actually occurs, our business, financial condition and results of operations could suffer, and you may lose all or part of your investment.

Risks Relating to the Notes and the Guarantees

The Issuer has no material assets and relies on us to make payment under the Notes

The Issuer was established specifically for the purpose of issuing indebtedness, including the Notes and will loan the proceeds from the issue of the Notes to us or to a company controlled by us. The Issuer does not and will not have any material assets other than amounts due to it from us in respect of such loan, and its ability to make payments under the Notes will depend on its receipt of timely payments from us in respect of such loan.

The Notes and the Guarantees are unsecured obligations

As the Notes and the Guarantees are unsecured obligations, the repayment of the Notes may be compromised if:

If any of these events were to occur, our assets may not be sufficient to make payments to the Issuer to pay amounts due on the Notes.

We may be unable to obtain and remit funds in foreign currencies

Our ability to satisfy our obligations to the Issuer or pursuant to the Guarantees depends in part upon the ability of our subsidiaries or affiliates in China to obtain and remit sufficient funds in foreign currencies to pay dividends to us and to repay intercompany loans. A significant portion of the revenues of our PRC subsidiaries are denominated in Renminbi. Our PRC subsidiaries must present certain documents to the State Administration of Foreign Exchange, its authorized branch or the designated foreign exchange bank, for approval before they can obtain and remit foreign currencies out of China (including, in the case of dividends, evidence that the relevant PRC taxes have been paid and, in the case of intercompany loans, evidence of the registration of the loan with the State Administration of Foreign Exchange).

Our credit ratings may not reflect all risks of your investments in the Notes

Our credit ratings are an assessment by rating agencies of our ability to pay our debts when due. Consequently, real or anticipated changes in our credit ratings will generally affect the market value of the Notes. These credit ratings may not reflect the potential impact of risks relating to the structure or marketing of the Notes. Agency ratings are not a recommendation to buy, sell or hold any security, and may be revised or withdrawn at any time by the issuing organization. Each agency’s rating should be evaluated independently of any other agency’s rating.

The insolvency laws of Hong Kong and other local insolvency laws may differ from those of other jurisdictions

Because we are incorporated under the laws of Hong Kong, any insolvency proceeding relating to us would likely involve Hong Kong insolvency laws, the procedural and substantive provisions of which may differ from comparable provisions of the local insolvency laws of jurisdictions with which you may be familiar.

S-10

A trading market for the Notes may not develop

The Notes are a new issue of securities for which there is currently no trading market. Application has been made to the HKSE for listing of, and permission to deal in, the Notes by way of debt issued to professional investors only. We and the Issuer have been advised that the underwriters intend to make a market in the Notes, but the underwriters are not obligated to do so and may discontinue such market-making activity at any time without notice. We cannot predict whether an active trading market for the Notes will develop or be sustained.

Investment in the Notes may subject you to foreign exchange risks

The Notes are denominated and payable in U.S. dollars. If you measure your investment returns by reference to a currency other than U.S. dollars, an investment in the Notes entails foreign exchange related risks, including possible significant changes in the value of the U.S. dollar relative to the currency by reference to which you measure your investment returns, due to, among other things, economic, political and other factors over which we have no control. Depreciation of the U.S. dollar against such currency could cause a decrease in the effective yield of the Notes below their stated coupon rates and could result in a loss when the return on the Notes is translated into such currency. In addition, there may be tax consequences for you as a result of any foreign exchange gains resulting from any investment in the Notes.

Payments on the Notes may, and payments under the Guarantee will, be subject to PRC withholding tax and the Issuer may redeem each series of Notes in whole at a redemption price equal to 100% of the principal amount plus accrued and unpaid interest in the event it is required to pay additional amounts in respect of PRC withholding tax in excess of 10% because it is treated as a PRC “resident enterprise”

In the event the Issuer is treated as a PRC “resident enterprise” under the PRC Enterprise Income Tax Law, it may be required to withhold PRC tax on interest payable to non-resident holders of Notes. As of the date of this prospectus supplement, the Company is treated as a PRC “resident enterprise” and is required to withhold PRC tax on interest payable to non-resident holders of Notes with respect to the Notes pursuant to the Guarantees. If the Issuer is required to withhold PRC tax from interest payments, or the Company is required to withhold PRC tax from payments it makes pursuant to the Guarantees, the Issuer or the Company will, subject to certain exceptions, be required to pay such additional amounts as will result in receipt by a holder of Notes of such amounts as would have been received by the holder of Notes had no such withholding been required.

As described under “Description of Debt Securities and Guarantees—Optional Tax Redemption” in the accompanying prospectus, in the event the Company or the Issuer is required to pay additional amounts as a result of certain changes in, or certain changes to the official interpretation or application of, tax law (including any change or interpretation or application that results in it being required to withhold tax in excess of 10% on interest payments as a result of the Issuer being treated as a PRC “resident enterprise” or on payments under the Guarantees), the Issuer may redeem each series of Notes in whole at a redemption price equal to 100% of the principal amount plus accrued and unpaid interest.

Non-resident holders of Notes may be subject to PRC taxes on disposition gains

If the Issuer is treated as a PRC “resident enterprise” under the Enterprise Income Tax Law and related implementation regulations in the future, any gain realized by non-resident enterprise holders of Notes from the disposition of the Notes may be regarded as derived from sources within the PRC and accordingly would be subject to 10% enterprise income tax, and any gain realized by non-resident individual holders of Notes from the disposition of the Notes may be regarded as derived from sources within the PRC and accordingly would be subject to 20% individual income tax. Applicable tax treaties may provide for lower tax rates.

S-11

Developments in other markets may adversely affect the market price of the Notes

The market price of the Notes may be adversely affected by declines in the international financial markets and world economic conditions. The market for Chinese securities is, to varying degrees, influenced by economic and market conditions in other markets, especially those in Asia. Although economic conditions are different in each country, investors’ reactions to developments in one country can affect the securities markets and the securities of issuers in other countries, including China. Since the subprime mortgage crisis in 2008, the international financial markets have experienced significant volatility. If similar developments occur in the international financial markets in the future, the market price of the Notes could be adversely affected.

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USE OF PROCEEDS

The aggregate proceeds from this offering, after deducting underwriting commissions and estimated offering expenses payable by the Issuer and us, are estimated to be approximately US$             million.    

The Issuer will loan the proceeds of this offering to us or a company controlled by us. The proceeds will be used in part to repay all or part of certain outstanding borrowings of our wholly-owned subsidiary Nexen Energy Capital Management U.S.A. Inc. Up to approximately US$500 million, or approximately             %, of the proceeds from this offering are expected to be used to repay the lenders that act as or are otherwise affiliated with certain underwriters of this offering. See “Underwriting (Conflicts of Interest).” The remaining proceeds from this offering, if any, will be used for general corporate purposes.

The foregoing represents our current intentions to use and allocate the proceeds of this offering based upon our present plans and business conditions. Our management, however, will have significant flexibility and discretion to apply the proceeds from the issuance of the Notes. If an unforeseen event occurs or business conditions change, we may use the proceeds from the issuance of the Notes differently than as described in this prospectus supplement.

In utilizing the proceeds from the issuance of the Notes, under PRC laws and regulations, we, as an offshore holding company, are permitted to provide funding to our PRC subsidiaries and consolidated affiliates only through loans or capital contributions and to other entities only through loans. Subject to satisfaction of applicable government registration and approval requirements, we may extend inter-company loans to our PRC subsidiaries and consolidated affiliates or make additional capital contributions to our PRC subsidiaries and consolidated affiliates to fund their capital expenditures or working capital. We cannot assure you that we will be able to obtain these government registrations or approvals on a timely basis, if at all.

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EXCHANGE RATE INFORMATION

China

The following table sets forth for the periods indicated, certain information concerning the exchange rates between Renminbi and U.S. dollars. The exchange rates reflect the exchange rates as set forth in the H.10 statistical release of the Board of Governors of the Federal Reserve System of the United States.

Note:

Hong Kong

The following table sets forth, for the periods indicated, certain information concerning the exchange rates between Hong Kong dollars and U.S. dollars. The exchange rates reflect the exchange rates as set forth in the H.10 statistical release of the Board of Governors of the Federal Reserve System of the United States.

Note:

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CAPITALIZATION

The following table sets forth our consolidated capitalization under IFRS as of December 31, 2017:

This table should be read in conjunction with, and is qualified in its entirety by reference to, our consolidated financial statements and the notes thereto incorporated by reference into this prospectus supplement.

Notes:

There has been no material change in our consolidated capitalization since December 31, 2017.

S-15

DESCRIPTION OF THE NOTES AND GUARANTEES

The following description is only a summary of the material terms of the Notes and the Guarantees and does not purport to be complete. The Notes will be issued pursuant to an indenture dated as of May 5, 2015 (the “Indenture”) among the Issuer, the Company and The Bank of New York Mellon as trustee (the “trustee”), initial paying and transfer agent and initial registrar. A form of the Indenture has been filed as an exhibit to the registration statement of which this prospectus supplement and the accompanying prospectus form a part. A copy of the Notes, the Guarantees and the Indenture will also be available for inspection at the corporate trust office of the trustee. You may also request copies of the Indenture from us at our address set forth under “Where You Can Find More Information About Us” in this prospectus supplement. The holders of each series of the Notes will be bound by, and be deemed to have notice of, all the provisions of the Indenture. The following summary of certain provisions of the Notes, the Guarantees and the Indenture is subject to and qualified in its entirety by reference to the detailed provisions of the Notes, the Guarantees and the Indenture and to the Trust Indenture Act of 1939, as amended. Terms and expressions used in this section and not otherwise defined shall have the meanings given to such terms in the Notes and the Indenture. This summary supplements the description of the debt securities in the accompanying prospectus and, to the extent it is inconsistent, replaces the description in the accompanying prospectus.

General

The 20             Notes will be issued by the Issuer in an initial aggregate principal amount of US$             and will mature on             , 20            , unless the 20             Notes are redeemed earlier pursuant to the respective terms thereof and of the Indenture. The 20             Notes are collectively referred to as the Notes.

The 20             Notes will bear interest at the rate of             % per annum. Interest on all series of Notes will accrue from             , 2018 or from and including the most recent Interest Payment Date (as defined below) to which interest has been paid or provided for, to and excluding the next Interest Payment Date or the maturity date, payable semi-annually in arrears on              and              in each year (each, an “Interest Payment Date”), commencing on             , 2018, to the persons in whose names the Notes are registered at the close of business (whether or not a Business Day) on              and             , respectively (each an “Interest Record Date”) immediately preceding an Interest Payment Date. Interest shall be calculated on the basis of a 360-day year consisting of twelve 30-day months.

In any case where the date of maturity of the principal of or interest on the Notes or the date fixed for redemption of the Notes is not a Business Day, then payment of principal or interest shall be made on the next succeeding Business Day, with the same force and effect as if made on the date of maturity or the date fixed for redemption, as the case may be, and no interest shall accrue for the period after such date. “Business Day” means a day in The City of New York, Hong Kong and the applicable place of payment other than a Saturday, Sunday or a day on which banking institutions are authorized or obligated by law or executive order to remain closed.

Notwithstanding the above, so long as the Global Note is held on behalf of Euroclear or Clearstream, each payment in respect of the Global Note will be made to the person shown as the holder in the Register at the close of business of the relevant clearing system on the Clearing System Business Day before the due date for such payments, where “Clearing System Business Day” means a weekday (Monday to Friday, inclusive) except December 25 and January 1.

The Notes shall be denominated in minimum principal amounts of US$200,000 and in integral multiples of US$1,000 in excess thereof.

The Notes will be the direct, unconditional, unsubordinated and unsecured obligations of the Issuer, and rank pari passu with all of its other unsecured and unsubordinated obligations (other than obligations preferred by applicable law) and senior in priority of payment and in all other respects to all of its other Indebtedness that is designated as subordinate or junior in right of payment to the Notes.

The Notes are unconditionally guaranteed as to the payment of the principal and interest in respect thereof and all other amounts payable under the Notes by the Issuer as evidenced by the Guarantee set forth in the

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indenture. Each Guarantee is the direct, unconditional, unsubordinated and unsecured obligation of the Company and will rank pari passu with all of its other unsecured and unsubordinated obligations (other than obligations preferred by applicable law) and senior in priority of payment and in all other respects to all of its other Indebtedness that is designated as subordinate or junior in right of payment to the Guarantee.

The principal of, interest on, and all other amounts payable under, the Notes will be payable at the Issuer’s office or agency (which initially will be the corporate trust office of The Bank of New York Mellon as paying agent, located at 101 Barclay Street, New York, NY 10286 United States of America), and the Notes may be exchanged or transferred at the Issuer’s office or agency (which initially will be the office of The Bank of New York Mellon as registrar, located at 101 Barclay Street, New York, NY 10286 United States of America) or, in each case, at such other location or locations as the Issuer, in consultation with the trustee, may designate. The principal of and interest on the Notes will be made by wire transfer or otherwise in immediately available funds and payable in U.S. dollars or in such other coin or currency of the United States of America as of the time of payment is legal tender for the payment of public and private debts. Payments of interest and principal with respect to interests in the global notes will be credited to the respective accounts of the holders of such interests with DTC or its participants, including Euroclear and Clearstream, Luxembourg. See “—Notes; Delivery and Form.”

Further Issues

The 20             Notes will be issued in an initial aggregate principal amount of US$            . The Company and the Issuer may, however, from time to time, without the consent of the holders of the Notes, create and issue pursuant to the Indenture, additional Notes of a series having the same terms and conditions under the Indenture as the previously outstanding Notes in all respects, except for issue date, issue price, and amount of the first payment of interest thereon. Additional Notes issued may be consolidated with and form a single series with the previously outstanding Notes of the relevant series; provided, however, that such additional Notes will not be issued under the same CUSIP, ISIN, Common Code or other identifying number as the outstanding Notes of that series unless such additional Notes are fungible with such outstanding Notes for U.S. federal income tax purposes.

Optional Redemption

Unless earlier redeemed in the limited circumstances set forth below, the 20             Notes will mature on             , 20             at a price equal to 100% of the principal amount thereof. Except as set forth in the accompanying prospectus or below, the Notes are not redeemable at the Issuer’s option.

The Company or the Issuer may, at the Company’s or the Issuer’s option, at any time and from time to time redeem, as applicable, the 20             Notes, in whole or in part, on not less than 30 nor more than 60 days’ prior notice mailed to the holders of such Notes, with a copy provided to the trustee and the paying agent. The applicable Notes will be redeemable at a redemption price equal to the greater of (1) 100% of the principal amount of the applicable Notes to be redeemed and (2) the sum of the present values of the remaining scheduled payments of principal and interest on the applicable Notes to be redeemed (not including interest accrued to the date of redemption), discounted to the date of redemption on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate plus              basis points in the case of the 20             Notes, plus accrued and unpaid interest on the applicable Notes to be redeemed, if any, to the date of redemption.

“Comparable Treasury Issue” means the United States Treasury security selected by an Independent Investment Banker that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of comparable maturity to the remaining term of the Notes to be redeemed. “Independent Investment Banker” means one of the Reference Treasury Dealers appointed by the Company.

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“Comparable Treasury Price” means, with respect to any redemption date, (1) the average of the Reference Treasury Dealer Quotations for such redemption date, after excluding the highest and lowest of such Reference Treasury Dealer Quotations, or (2) if the Company obtains fewer than three such Reference Treasury Dealer Quotations, the average of all quotations obtained.

“Reference Treasury Dealer” means each of any three investment banks of recognized standing that is a primary U.S. government securities dealer in the United States, selected by the Company in good faith.

“Reference Treasury Dealer Quotations” means, with respect to each Reference Treasury Dealer and any redemption date, the average, as determined by the Company, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Company by such Reference Treasury Dealer as of 5:00 p.m., New York City time, on the third Business Day preceding such date of redemption.

“Treasury Rate” means, with respect to any date of redemption, the rate per annum equal to the semi-annual equivalent yield to maturity (computed as of the third Business Day immediately preceding such redemption date) of the Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such date of redemption.

Notification Regarding Acquisition of Notes by Related Persons

By acceptance of its Note or its interest in a Note, each holder or beneficial owner that is considered a “foreign person” that is a “related party” with respect to the Issuer or its shareholder under Section 59A(g) of the Internal Revenue Code of 1986, as amended (the “Code”) agrees to notify the Issuer of its status as such within 30 days of the acquisition of Notes or a beneficial interest therein (or if such holder or beneficial owner becomes a related party at a later date, within 30 days from such later date) so that Issuer or its shareholder can comply with their reporting requirements under Code section 6038A. Generally, a foreign person that is a related party for these purposes includes any non-U.S. person that is actually or constructively a 25% owner of the Issuer (by vote or value), a person related (within the meaning of Section 267(b) or Section 707(b)(1) of the Code) to the Issuer or a 25% owner thereof, or any person related to the Issuer within the meaning of Section 482 of the Code.

Notification to the Issuer shall be delivered, mailed or emailed to Nexen Energy ULC at 801 7th Ave SW, Calgary, AB, Canada T2P 3P7 or at Governance@nexencnoocltd.com; Attention: Treasurer.

No Sinking Fund

The Notes will not be entitled to the benefit of any sinking fund.

Notes; Delivery and Form

The statements set forth herein include summaries of certain rules and operating procedures of DTC, Euroclear and Clearstream, Luxembourg which will affect transfers of interests in the global notes.

The Notes will be represented by one or more global notes, fully registered without interest coupons, which will be deposited with The Bank of New York Mellon, as custodian for DTC (in such capacity, the “Custodian”) and registered in the name of Cede & Co., as nominee of DTC, for the accounts of its participants, including Euroclear and Clearstream, Luxembourg.

The Notes will be issued in minimum denominations of US$200,000 and in integral multiples of US$1,000 in excess of that amount.

Investors may hold their interests in the global notes directly through DTC, Clearstream, Luxembourg or Euroclear, as the case may be, if they are participants in such systems, or indirectly through organizations which

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are participants in such systems. Clearstream, Luxembourg and Euroclear will hold interests in the global notes on behalf of their participants through customers’ securities accounts in their respective names on the books of their respective depositaries, which are participants in DTC.

Transfers between participants in DTC (the “Participants”) will be effected in the ordinary way in accordance with DTC rules. Transfers between participants in Clearstream, Luxembourg and Euroclear (“Clearstream Participants” and “Euroclear Participants,” respectively) will be effected in the ordinary way in accordance with their respective rules and operating procedures.

Persons who are not Participants may beneficially own interests in the global notes held by DTC only through Participants or Indirect Participants (as defined below) (including Euroclear and Clearstream, Luxembourg). So long as Cede & Co., as the nominee of DTC, is the registered owner of the global notes, Cede & Co., for all purposes, will be considered the sole holder of such Notes.

Payment of interest on and principal of the global notes will be made to Cede & Co., the nominee for DTC, as the registered owner of the global notes by wire transfer of immediately available funds. None of the Company, the Issuer, the trustee or paying agent will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in the global notes or for maintaining, supervising or reviewing any records relating to such beneficial ownership interest.

The Issuer has been informed by DTC that, upon receipt of any payment of interest on or the redemption price of the global notes, DTC will credit Participants’ accounts with payments in amounts proportionate to their respective beneficial interests in the global notes as shown on the records of DTC. Payments of interest on and principal of the Notes held through Clearstream, Luxembourg or Euroclear will be credited to the cash accounts of Clearstream Participants or Euroclear Participants, as the case may be, in accordance with the relevant system’s rules and procedures. Payments by Participants to owners of beneficial interests in the global notes held through such Participants will be the responsibility of such Participants, as is the case with securities held by broker-dealers, either directly or through nominees, for the accounts of customers and registered in “street name.”

Because DTC can only act on behalf of Participants, who in turn act on behalf of Indirect Participants and certain banks, the ability of a person having a beneficial interest in the global notes to pledge such interest to persons or entities that do not participate in the DTC system, or to otherwise take actions in respect of such interest, may be affected by the lack of a physical certificate. The laws of some states require that certain purchasers of securities take physical delivery of such securities in definitive form. Such limits and such laws may impair the ability to own, transfer or pledge beneficial interests in the global notes.

So long as the Notes are represented by global notes and such global notes are held on behalf of DTC or any other clearing system, such clearing system or its nominee will be considered the sole holder of the Notes represented by the applicable global notes for all purposes under the indenture, including, without limitation, obtaining consents and waivers thereunder, and none of the Company, the Issuer or the trustee shall be affected by any notice to the contrary. None of the Company, the Issuer, the trustee or the registrar shall have any responsibility or obligation with respect to the accuracy of any records maintained by any clearing system or any Participant of such clearing system. The clearing systems will take actions on behalf of their Participants (and any such Participants will take actions on behalf of any Indirect Participants) in accordance with their standard procedures. To the extent that any clearing system acts upon the direction of the holders of the beneficial interests in the applicable global note and such beneficial holders give conflicting instructions, the applicable clearing system may take conflicting actions in accordance with such instructions.

DTC has advised the Issuer that it will take any action permitted to be taken by a holder of Notes (including, without limitation, the presentation of Notes for exchange) only at the direction of one or more Participants and only in respect of the principal amount of the Notes represented by the global note as to which such Participant or Participants have given such direction.

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Clearstream, Luxembourg or Euroclear, as the case may be, will take any action permitted to be taken by a holder of Notes (including, without limitation, the presentation of Notes for exchange) on behalf of a Clearstream Participant or a Euroclear Participant only in accordance with its relevant rules and procedures and subject to its ability to effect such actions through DTC.

Although DTC, Clearstream, Luxembourg and Euroclear have agreed to the foregoing procedures in order to facilitate transfers of interests in the global notes among Participants of DTC, Clearstream, Luxembourg and Euroclear, they are under no obligation to perform or continue to perform such procedures, and such procedures may be discontinued at any time. None of the Company, the Issuer or the trustee will have any responsibility for the performance by DTC, Clearstream, Luxembourg and Euroclear, or their respective Participants or Indirect Participants, of their respective obligations under the rules and procedures governing their operations.

DTC is a limited purpose trust company organized under the laws of the State of New York, a member of the Federal Reserve System, a “clearing corporation” within the meaning of the Uniform Commercial Code and a “Clearing Agency” registered pursuant to the provisions of Section 17A of the U.S. Securities Exchange Act of 1934. DTC was created to hold securities for its Participants and facilitate the clearance and settlement of securities transactions between Participants through electronic book-entry changes in accounts of its Participants, thereby eliminating the need for the physical movement of certificates. Participants include securities brokers and dealers, banks, trust companies and clearing corporations and may include certain other organizations. Indirect access to the DTC system is available to others such as banks, brokers, dealers and trust companies that clear through or maintain a custodial relationship with a Participant, either directly or indirectly (“Indirect Participants”).

Euroclear and Clearstream, Luxembourg hold securities for participating organizations. They also facilitate the clearance and settlement of securities transactions between their respective Participants through electronic book-entry changes in the accounts of such Participants. Euroclear and Clearstream, Luxembourg provide various services to their Participants, including the safekeeping, administration, clearance, settlement, lending and borrowing of internationally traded securities. Euroclear and Clearstream, Luxembourg interface with domestic securities markets. Euroclear and Clearstream, Luxembourg participants are financial institutions such as underwriters, securities brokers and dealers, banks, trust companies and certain other organizations. Indirect access to Euroclear or Clearstream, Luxembourg is also available to others such as banks, brokers, dealers and trust companies that clear through or maintain a custodial relationship with a Euroclear or Clearstream, Luxembourg Participant, either directly or indirectly.

The information in this section concerning DTC and DTC’s book-entry system, as well as information regarding Euroclear and Clearstream, Luxembourg, has been obtained from sources that we believe to be reliable, but neither we nor any underwriter takes responsibility for its accuracy or completeness. We assume no responsibility for the performance by DTC, Euroclear, Clearstream, Luxembourg or their respective Participants or Indirect Participants of their respective obligations, including obligations that they have under the rules and procedures that govern their operations.

Individual Notes

If DTC is at any time unwilling or unable to continue as depositary and a successor depositary is not appointed by the Issuer within 90 days or if there shall have occurred and be continuing an Event of Default (as described in the attached Prospectus) with respect to the Notes, the Issuer will issue individual Notes in certificated, fully registered form in exchange for the global notes. The holder of such individual Notes in certificated form may transfer or exchange such Notes by surrendering them at the office of the registrar.

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TAXATION

The statements herein regarding taxation are based on the laws in force as of the date of this prospectus supplement and are subject to any changes in laws occurring after such date, which could be made on a retroactive basis. The following summary does not purport to be a comprehensive description of all of the tax considerations that may be relevant to a decision of a prospective purchaser to acquire or dispose of the Notes and does not purport to deal with the tax consequences applicable to all categories of investors, some of which (such as dealers in securities or commodities) may be subject to special rules. Prospective purchasers are advised to consult their own tax advisors concerning the overall tax consequences of the purchase, ownership and disposition of the Notes.

The PRC

Pursuant to the Notice on Issued about the Determination of Chinese-Controlled Enterprises Registered Abroad as Resident Enterprises on the Basis of Actual Management (the “Notice”) issued by the State Administration of Taxation of the PRC (the “SAT”) on April 22, 2009, the Enterprise Income Tax Law and the Implementation Rules, which both took effect from January 1, 2008, enterprises established outside of the PRC whose “de facto management bodies” are located in the PRC are considered Chinese resident enterprises (“CREs”). According to a formal reply from the SAT in October 18, 2010, the Company is deemed as a CRE pursuant to the provisions of the Notice, the Enterprise Income Tax Law and the Implementation Rules. Accordingly, the Company is obliged to withhold PRC income tax of up to 10% on payments of interest under the Guarantees to non-PRC enterprise holders of Notes and of up to 20% on payments of interest to non-PRC individual holders of Notes. Applicable tax treaties may provide for lower rates.

As of the date of this prospectus supplement, the Issuer has not been informed by the SAT that it is deemed a CRE. If the Issuer is deemed to be a CRE for enterprise income tax purposes, among other things, the Issuer would be subject to the PRC enterprise income tax at the rate of 25% on its worldwide income. Furthermore, the Issuer would be obligated to withhold PRC income tax of up to 10% on payments of interest to non-PRC enterprise holders of Notes and of up to 20% on payments of interest to non-PRC individual holders of Notes. Under the terms of the Notes, if the Issuer is treated as a PRC resident enterprise by the SAT and is required to withhold tax on payments due under the Notes, it will, subject to certain exceptions, be required to pay such additional amounts as will result in receipt by each holder of Notes of such amounts as would have been received by such holder of Notes had no such withholding been required. See “Description of the Debt Securities and Guarantees—Additional Amounts” in the accompanying prospectus. In addition, if the Issuer is treated as a CRE, any gain realized by non-resident enterprise holders of Notes from the disposition of the Notes may be regarded as derived from sources within the PRC and accordingly would be subject to 10% enterprise income tax, and any gain realized by non-resident individual holders of Notes from the disposition of the Notes may be regarded as derived from sources within the PRC and accordingly would be subject to 20% individual income tax. Applicable tax treaties may provide for lower tax rates with respect to any PRC tax liability.    

Hong Kong

The following summary contains a description of the principal tax laws of Hong Kong, as in effect on the date of this prospectus supplement, and is subject to any change in the tax laws of Hong Kong that may come into effect after such date (which may have retroactive effect).

No Hong Kong tax will be required to be withheld from payments of principal or interest on the Notes or payments by us in respect thereof under the Guarantee.

Interest on the Notes is not subject to Hong Kong profits tax, except where the interest is received by or accrued to a financial institution (as defined in the Inland Revenue Ordinance) through or from the carrying on by the financial institution of its business in Hong Kong.

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Hong Kong profits tax is not chargeable in respect of capital gains from the sale of property such as the Notes. However, trading profits from the sale, disposal or redemption of the Notes by persons carrying on a trade, profession or business in Hong Kong where such profits are derived from or arise in Hong Kong from such, trade, profession or business are chargeable to profits tax currently at the rate of 16.5% on corporations and 15% on individuals. Profits from the sale of Notes affected on the HKSE will be considered to be derived from or arise in Hong Kong. Liability for Hong Kong profits tax would thus arise in respect of trading gains from the sale, disposal or redemption of the Notes realized by persons carrying on a business of trading or dealing in securities in Hong Kong.

No Hong Kong stamp duty is payable on the sale, purchase or other disposal of bonds or notes denominated otherwise than in the currency of Hong Kong, provided that the bonds or notes are not redeemable, and may not at the option of any person be redeemed, in the currency of Hong Kong. Therefore, a sale or purchase or other disposal of the Notes will not be subject to Hong Kong stamp duty.

U.S. Federal Income Taxation

The following are material U.S. federal income tax consequences to the U.S. Holders and Non-U.S. Holders described below of owning and disposing of Notes purchased in this offering at the “issue price,” which for each series of Notes will equal the first price at which a substantial amount of such Notes is sold to the public (not including bond houses, brokers or similar persons acting in the capacity of underwriters, placement agents or wholesalers), and held as capital assets for U.S. federal income tax purposes.

This discussion does not describe all of the tax consequences that may be relevant to you in light of your particular circumstances, including the possible effects of Section 451(b) of the Internal Revenue Code of 1986, as amended (the “Code”), alternative minimum tax and Medicare contribution tax consequences, and differing tax consequences that may apply if you are, for instance:

If an entity or arrangement treated as a partnership for U.S. federal income tax purposes holds the Notes, the tax treatment of a partner will generally depend upon the status of the partner and the activities of the partnership. A partnership or a partner of a partnership holding the Notes should consult its own tax advisor.

This summary is based on the Code, administrative pronouncements, judicial decisions and final, temporary and proposed Treasury Regulations, as well as the income tax treaty between the United States and the PRC (the “Treaty”), all as of the date hereof, any of which may be repealed, revoked or modified (possibly with retroactive effect) so as to result in U.S. federal income tax consequences different from those discussed below. Persons considering the purchase, ownership or disposition of Notes should consult their tax advisers concerning the application of the U.S. federal income tax laws to their particular situations, as well as any tax consequences arising under the laws of any state, local, or non-U.S. taxing jurisdiction.

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Tax consequences to U.S. Holders

This section applies only to a U.S. Holder of a Note. You are a U.S. Holder for purposes of this discussion if for U.S. federal income tax purposes you are a beneficial owner of a Note and are:

It is expected, and this discussion assumes, that the Notes of each series will be issued without “original issue discount” for U.S. federal income tax purposes. If, however, the principal amount of the Notes of any series exceeds their issue price by a specified de minimis amount or more, as determined under applicable Treasury Regulations, a U.S. Holder will be required to include that excess in income as original issue discount, as it accrues, in accordance with a constant-yield method based on a compounding of interest, before the receipt of cash attributable to this income.

Payments of interest. Stated interest paid on a Note will be taxable to you as ordinary interest income at the time it accrues or is received, in accordance with your method of accounting for U.S. federal income tax purposes. Interest (including payments by the Company under the Guarantee) will generally be treated as U.S.-source income for U.S. federal income tax purposes.

Sale or other taxable disposition of the Notes. You will generally recognize capital gain or loss upon the sale or other disposition of the Notes in an amount equal to the difference between the amount realized and your tax basis in the Notes. The amount realized will not include any amount attributable to accrued interest, which will be treated as described above under “—Payments of interest.” Your tax basis will generally equal your cost of the Notes. Gain or loss, if any, will be long-term capital gain or loss if at the time of sale or other disposition the Notes have been held for more than one year. The deductibility of capital losses is subject to limitations. Any gain or loss will generally be U.S.-source gain or loss for foreign tax credit purposes.

As described in “Taxation—The PRC,” gains from the disposition of Notes may be subject to PRC tax if the Issuer is considered to be a Chinese resident enterprise. If that were the case, your amount realized would include the gross amount of proceeds. Although gain realized on disposition of Notes would generally be characterized as U.S.-source income, if you are eligible for the benefits of the Treaty, you may be able to treat the gain as foreign-source gain for foreign tax credit purposes if the gain is subject to PRC tax. You should consult your tax adviser regarding the creditability of any PRC taxes on disposition of Notes.

Backup withholding and information reporting. You will be subject to information reporting requirements with respect to payments on the Notes and proceeds from a sale of the Notes if paid within the United States or through certain U.S.-related financial intermediaries, unless an exception applies. In addition, certain U.S. Holders may be subject to backup withholding in respect of those payments if you do not provide your taxpayer identification numbers and certify that you are not subject to backup withholding, or provide proof of an applicable exemption. Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be refunded or credited against your U.S. federal income tax liability, provided that the required information is timely furnished to the Internal Revenue Service (the “IRS”).

Tax consequences to Non-U.S. Holders

This section applies only to a Non-U.S. Holder. You are a Non-U.S. Holder for purposes of this discussion if for U.S. federal income tax purposes you are a beneficial owner of a Note and are:

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You are not a Non-U.S. Holder for purposes of this discussion if you are a nonresident alien individual present in the United States for 183 days or more in the taxable year of disposition, or a former citizen or former resident of the United States, in either of which cases you should consult your tax adviser regarding the U.S. federal income tax consequences of owning and disposing of a Note.

Payments on the Notes. Subject to the discussions below under “Backup withholding and information reporting” and “FATCA,” payments of principal or interest to you on a Note will generally not be subject to U.S. federal income tax or withholding tax, provided that, in the case of interest:

If you cannot satisfy one of the first three requirements described above and interest on the Notes is not exempt from withholding because it is effectively connected with your conduct of a trade or business in the United States as described below, payments of interest on the Notes will be subject to withholding tax at a rate of 30%, or the rate specified by an applicable income tax treaty.

Sale or other taxable disposition of the Notes. You generally will not be subject to U.S. federal income or withholding tax on gain realized on a sale, redemption or other taxable disposition of the Notes, unless the gain is effectively connected with your conduct of a trade or business in the United States as described below, although any amounts attributable to accrued interest will be treated as described above under “—Payments on the Notes.”

Effectively connected income. If interest or gain from a Note is effectively connected with your conduct of a trade or business in the United States (and, if required by an applicable income tax treaty, is attributable to a U.S. permanent establishment or fixed base maintained by you), you will generally be taxed in the same manner as a U.S. Holder. In this case, you will be required to provide a properly executed IRS Form W-8ECI in order to claim an exemption from withholding tax on interest. You should consult your tax adviser with respect to other U.S. tax consequences of owning and disposing of Notes, including the possible imposition of a branch profits tax at a rate of 30% (or a lower treaty rate) if you are a corporation.

Backup withholding and information reporting. Information returns will be filed with the IRS in connection with payments of interest on the Notes. Unless you comply with certification procedures to establish that you are not a United States person, information returns may also be filed with the IRS in connection with payment of the proceeds from a sale or other disposition of a Note, and you may be subject to backup withholding on payments on the Notes or on the proceeds from a sale or other disposition of the Notes. The certification of non-U.S. status on an IRS Form W-8BEN or IRS Form W-8BEN-E (as applicable) described above will satisfy the certification requirements necessary to avoid backup withholding as well. Backup withholding is not an additional tax. The amount of any backup withholding from a payment to you will be allowed as a credit against your U.S. federal income tax liability and may entitle you to a refund, provided that the required information is timely furnished to the IRS.

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FATCA

Legislation commonly referred to as “FATCA” generally imposes a withholding tax of 30% on payments to certain non-U.S. entities (including financial intermediaries) with respect to certain financial instruments, unless various U.S. information reporting and due diligence requirements have been satisfied. An intergovernmental agreement between the United States and the non-U.S. entity’s jurisdiction may modify these requirements.

Withholding under these rules (if applicable) applies to payments of interest on the Notes and, after December 31, 2018, to payments of gross proceeds of the sale, exchange or retirement of the Notes. You should consult your tax advisers regarding the potential application of FATCA to the Notes.

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UNDERWRITING (CONFLICTS OF INTEREST)

Subject to the terms and conditions set forth in an underwriting agreement dated             , 2018 among us, the Issuer and the underwriters named below, and the underwriting agreement standard provisions filed as an exhibit to the registration statement of which this prospectus supplement and the accompanying prospectus form a part, the Issuer has agreed to sell to each underwriter, and each underwriter has agreed, severally and not jointly, to purchase from the Issuer, the principal amount of the Notes of each series set forth opposite its name below:

The underwriting agreement provides that the obligations of the underwriters to purchase the Notes are subject to certain conditions, including, among others, the receipt by the underwriters of documentation related to the issuance and settlement of the Notes, officer’s certificates and legal opinions. The underwriters must purchase all the Notes if they purchase any of the Notes. The underwriters reserve the right to withdraw, cancel or modify offers to investors and to reject orders in whole or in part. The underwriting agreement may be terminated by the underwriters in certain circumstances prior to the delivery of and payment for the Notes.

We and the Issuer have agreed to indemnify the underwriters against certain liabilities, including liabilities under the U.S. Securities Act of 1933, or to contribute to payments the underwriters may be required to make in respect of those liabilities.

The underwriters propose initially to offer part of the Notes of each series directly to the public at the offering prices set forth on the cover page of this prospectus supplement. After the initial offering of the Notes, the offering price may be changed.

Certain of the underwriters are not broker-dealers registered with the SEC. Therefore, to the extent that they intend to make any offers or sales of Notes in the United States, they will do so only through one or more registered broker-dealers in compliance with applicable securities laws and regulations and FINRA rules. Goldman Sachs (Asia) L.L.C. will offer the Notes in the United States through its registered broker-dealer affiliate Goldman, Sachs & Co. LLC, The Hongkong and Shanghai Banking Corporation Limited will offer the Notes in the United States through its registered broker-dealer affiliate HSBC Securities (USA) Inc. and Société Générale will offer the Notes in the United States through its registered broker-dealer affiliate SG Americas Securities, LLC.

Due to certain restrictions imposed under the US Bank Holding Company Act of 1956, none of ICBC International Securities Limited and its affiliates may underwrite, subscribe, agree to purchase or procure purchasers to purchase Notes that are offered or sold in the United States. None of ICBC International Securities Limited nor its affiliates will procure purchasers to purchase Notes that may be offered or sold in the United States. ICBC International Securities Limited and its affiliates shall offer and sell Notes constituting part of their allotment solely outside the United States in accordance with Regulation S under the Securities Act.

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If a jurisdiction requires that the offering be made by a licensed broker or dealer and the underwriters or any affiliate of the underwriters is a licensed broker or dealer in that jurisdiction, the offering shall be deemed to be made by that underwriter or its affiliate on behalf of the Issuer in such jurisdiction.

The following table shows the underwriting discounts that we will pay to the underwriters in connection with this offering:

Expenses associated with this offering to be paid by us, other than underwriting commissions and discounts, are estimated to be US$            .

New Issue of Notes

The Notes are a new issue of securities with no established trading market. Application has been made to the HKSE for listing of, and permission to deal in, the Notes by way of debt issue to professional investors only. We have been advised by the underwriters that they presently intend to make a market in the Notes after completion of the offering. However, they are under no obligation to do so and may discontinue any market-making activities at any time without any notice. We cannot assure the liquidity of the trading market for the Notes. If an active trading market for the Notes does not develop, the market price and liquidity of the Notes may be adversely affected. If the Notes are traded, they may trade at a discount from their initial offering price, depending on prevailing interest rates, the market for similar securities, our operating performance and financial condition, general economic conditions and other factors.

Settlement

We expect that delivery of the Notes will be made to investors on or about the closing date specified on the cover page of this prospectus supplement, which will be the fifth business day following the date of the pricing of the Notes (such settlement being referred to as “T+5”). Under Rule 15c6-1 under the U.S. Securities Exchange Act of 1934, trades in the secondary market are generally required to settle in two business days, unless the parties to any such trade expressly agree otherwise. Accordingly, purchasers who wish to trade Notes on the date of pricing or on the next succeeding business day will be required, by virtue of the fact that the Notes will initially settle in T+5, to specify an alternate settlement arrangement at the time of any such trade to prevent a failed settlement. Purchasers of the Notes who wish to trade the Notes prior to their date of delivery hereunder should consult their advisors.

No Sales of Similar Securities

We and the Issuer have each agreed that we will not, for a period of 30 days after the date of closing (which is expected to be the fifth business day following the date of this prospectus supplement), without first obtaining the prior written consent of the underwriters, offer, sell or contract to sell, or otherwise dispose of, directly or indirectly, or announce the offering of, any debt securities issued or guaranteed by us or the Issuer having a tenor of more than one year, except for the Notes sold to the underwriters pursuant to the underwriting agreement and any loans, including bilateral or syndicated loans or club deals. The underwriters in their sole discretion may release any of the securities subject to these lock-up agreements at any time without notice.

Short Positions and Stabilizing Transactions

In connection with the offering, each underwriter (or its affiliates) may purchase and sell the Notes in the open market. These transactions may include short sales, purchases on the open market to cover positions created

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by short sales and stabilizing purchases. Short sales involve the sale by an underwriter of a greater principal amount of the Notes than they are required to purchase in the offering. An underwriter must close out any short position by purchasing the Notes in the open market. A short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the Notes in the open market after pricing that could adversely affect investors who purchase in the offering. Stabilizing transactions involve bids to purchase the Notes so long as the stabilizing bids do not exceed a specified maximum.

Similar to other purchase transactions, an underwriter’s purchases to cover the syndicate short sales and stabilizing purchases may have the effect of raising or maintaining the market price of the Notes or preventing or retarding a decline in the market price of the Notes. As a result, the price of the Notes may be higher than the price that might otherwise exist in the open market.

None of us, the Issuer, or any of the underwriters makes any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the Notes. In addition, none of us, the Issuer, or any of the underwriters makes any representation that the underwriters will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice at any time. No assurance can be given as to the liquidity of, or the trading market for, the Notes.

The address of Bank of China Limited is 7th Floor, Bank of China Tower, 1 Garden Road, Central, Hong Kong. The address of BOCI Asia Limited is 26th Floor, Bank of China Tower, 1 Garden Road, Central, Hong Kong. The address of Citigroup Global Markets Inc. is 388 Greenwich Street, New York, New York 10013, United States of America. The address of Credit Suisse Securities (USA) LLC is Eleven Madison Avenue, New York, New York 10010, United States of America. The address of Goldman Sachs (Asia) L.L.C. is 68th Floor, Cheung Kong Center, 2 Queen’s Road Central, Hong Kong. The address of The Hongkong and Shanghai Banking Corporation Limited is Level 17, HSBC Main Building, 1 Queen’s Road Central, Hong Kong. The address of J.P. Morgan Securities LLC is 383 Madison Avenue, New York, New York 10179, United States of America. The address of ICBC International Securities Limited is 37/F ICBC Tower, 3 Garden Road, Hong Kong. The address of Mizuho Securities USA LLC is 320 Park Avenue – 12th Floor, New York, New York 10022, United States of America. The address of Natixis Securities Americas LLC is 1251 Avenue of the Americas, New York, New York 10020, United States of America. The address of Société Générale is 34/F Three Pacific Place, 1 Queen’s Road East, Hong Kong.

Sales Outside the United States

Notice to Prospective Investors in the EEA

The Notes which are the subject of the offering contemplated by this prospectus supplement have not been offered, sold or otherwise made available and will not be offered, sold or otherwise made available in relation thereto to any retail investor in the European Economic Area. For the purposes of this provision, the expression “retail investor” means a person who is one (or more) of the following:

Notice to Prospective Investors in Italy

The offering of the Notes has not been registered with the Italian Securities and Exchange Commission (Commissione Nazionale per le Società e la Borsa, the “CONSOB”) pursuant to Italian securities legislation and, accordingly, no Notes may be offered, sold or delivered, nor may copies of this prospectus supplement, the

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accompanying prospectus, or any other document relating to the Notes be distributed in the Republic of Italy (“Italy”), except:

Moreover, and subject to the foregoing, any offer, sale or delivery of the Notes or distribution of copies of this prospectus supplement, the accompanying prospectus, or any other document relating to the Notes in Italy under (i) or (ii) above must be:

Any investor purchasing the Notes in the offering is solely responsible for ensuring that any offer or resale of the Notes it purchased in the offering occurs in compliance with applicable Italian laws and regulations.

Notice to Prospective Investors in the United Kingdom

No invitation or inducement to engage in investment activity (within the meaning of Section 21 of the Financial Services and Markets Act 2000 (the “FSMA”)) received by the underwriters in connection with the issue or sale of any Notes may be communicated in circumstances in which Section 21(1) of the FSMA does not apply to us or the Issuer. All applicable provisions of the FSMA must be complied with respect to anything done or to be done by underwriters in relation to the Notes in, from or otherwise involving the United Kingdom.

Notice to Prospective Investors in Hong Kong

You are advised to exercise caution in relation to the offer. If you are in any doubt about any of the contents of this document, you should obtain independent professional advice. The Notes are only available in Hong Kong or to persons resident in Hong Kong who are (a) “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) acquiring the Notes in circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance. Each purchaser of the Notes in the United States who is a resident of Hong Kong, by accepting delivery of this prospectus supplement and the accompanying prospectus, will be deemed to have represented, agreed and acknowledged that (a) it is a “professional investor” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) it is acquiring the Notes in circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance.

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No advertisement, invitation or document relating to the Notes, which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) has been issued or had in possession for the purposes of issue or will be issued or had in possession for the purposes of issue, whether in Hong Kong or elsewhere, other than with respect to Notes which are or are intended to be disposed of only to persons outside of Hong Kong or only to “professional investors” as defined in the Securities and Futures Ordinance and any rules made under that Ordinance.

Notice to Prospective Investors in Japan

The Notes offered in this prospectus supplement have not been registered under the Financial Instruments and Exchange Act of Japan (Act No. 25 of 1948, as amended, the “Financial Instruments and Exchange Act”). The Notes have not been offered or sold and will not be offered or sold, directly or indirectly, in Japan or to or for the account of any resident of Japan (which term as used herein means any person resident in Japan, including any corporation or other entity organized under the laws of Japan), or to others for re-offering or re-sale, directly or indirectly, in Japan or to, or for the benefit of, any resident of Japan except (i) pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the Financial Instruments and Exchange Act and (ii) in compliance with any other applicable requirements of Japanese laws and regulations.

Notice to Prospective Investors in the PRC

This prospectus supplement and the accompanying prospectus do not constitute a public offer of the Notes, whether by way of sale or subscription, in the PRC. Except as permitted by the securities laws of the PRC, the Notes are not being offered or sold and may not be offered or sold, directly or indirectly, in the PRC to or for the benefit of, legal or natural persons of the PRC.

Notice to Prospective Investors in Singapore

This prospectus supplement and the accompanying prospectus have not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus supplement, the accompanying prospectus, and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the Notes may not be circulated or distributed, nor may the Notes be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore (the “SFA”), (ii) to a relevant person pursuant to Section 275(1), or any person pursuant to Section 275(1A), and in accordance with the conditions specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA, in each case subject to compliance with conditions set forth in the SFA.

Where the Notes are subscribed or purchased under Section 275 of the SFA by a relevant person which is:

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Other Relationships

The underwriters and their affiliates have in the past engaged, and may in the future engage, in transactions with and perform services, including financial advisory and investment banking services, for us, the Issuer and our respective affiliates in the ordinary course of business, for which they received or will receive customary fees and expenses. We, the Issuer and our respective affiliates may enter into hedging or other derivative transactions as part of our risk management strategies with one or more of the underwriters, which may include transactions relating to our obligations under the Notes. Our and the Issuer’ obligations under these transactions may be secured by cash or other collateral.

The underwriters or certain of their affiliates may purchase the Notes and be allocated the Notes for asset management and/or proprietary purposes but not with a view to distribution.

The underwriters or their respective affiliates may purchase the Notes for their own account and enter into transactions, including credit derivatives, such as asset swaps, repackaging and credit default swaps relating to the Notes and/or other securities of us, the Issuer or our respective subsidiaries or associates at the same time as the offer and sale of the Notes or in secondary market transactions. Such transactions would be carried out as bilateral trades with selected counterparties and separately from any existing sale or resale of the Notes to which this prospectus supplement relates (notwithstanding that such selected counterparties may also be purchasers of the Notes). Certain of the underwriters or their affiliates that have a lending relationship with us routinely hedge their credit exposure to us consistent with their customary risk management policies. Typically, such underwriters and their affiliates would hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities, including potentially the Notes offered hereby. Any such short positions could adversely affect future trading prices of the Notes offered hereby. The underwriters and their respective affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Certain of the underwriters or their affiliates have a lending relationship with us. In connection with certain outstanding borrowings of our wholly-owned subsidiary Nexen Energy Capital Management U.S.A. Inc., Citibank, N.A., JPMorgan Chase Bank, N.A. and several other financial institutions acted as original lenders. The proceeds from this offering will be applied to pay a portion of our borrowings under these loan facilities, and as a result, the affiliates of Citigroup Global Markets Inc. and J.P. Morgan Securities LLC named above may receive a portion of the net proceeds from the sales of the Notes.

Conflicts of Interest

Because Citigroup Global Markets Inc. and J.P. Morgan Securities LLC are underwriters in this offering and their affiliates named above may receive more than 5% of the net proceeds from the sales of the Notes, a “conflict of interest” is deemed to exist under FINRA Rule 5121. Accordingly, this offering will be made in compliance with the applicable provisions of FINRA Rule 5121 and such underwriters will not sell any of the Notes to discretionary accounts without the specific written approval of the account holder.

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LEGAL MATTERS

Certain legal matters with respect to the Notes and the Guarantees will be passed upon for us by Davis Polk & Wardwell LLP, New York, New York, as to matters of United States federal securities and New York State law, by Davis Polk & Wardwell, Hong Kong, as to matters of Hong Kong law, respectively. Certain legal matters with respect to the Notes and the Guarantees will be passed upon for the underwriters by Linklaters, as to matters of United States federal securities and New York State law, and by Jun He Law Offices Beijing, China, as to matters of PRC law.

EXPERTS

The consolidated financial statements incorporated in this prospectus supplement by reference from the Company’s Annual Report on Form 20-F for the year ended December 31, 2017, and the effectiveness of the Company’s internal control over financial reporting as of December 31, 2017 have been audited by Deloitte Touche Tohmatsu, an independent registered public accounting firm, as stated in their reports, which are incorporated herein by reference. Such consolidated financial statements have been so incorporated in reliance upon the reports of such firm given upon their authority as experts in accounting and auditing.

Certain reserve information relating to our oil and gas reserves included in our annual report on Form 20-F for the year ended December 31, 2017 was based in part upon reserve reports prepared by independent consultants Ryder Scott Company, L.P., Gaffney, Cline & Associates (Consultants) Pte Ltd., RPS, McDaniel & Associates Consultants Ltd. and DeGolyer and MacNaughton. We have incorporated this information in this prospectus supplement by reference to such annual report and included certain of this information in this prospectus supplement in reliance on the authority of such firms as experts in estimating proved oil and gas reserves.

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WHERE YOU CAN FIND MORE INFORMATION ABOUT US

This prospectus supplement is part of a registration statement that we filed with the SEC. The registration statement, including the attached exhibits, contains additional relevant information about us. The rules and regulations of the SEC allow us to omit some of the information included in the registration statement from this prospectus supplement. In addition, we are subject to the periodic reporting and other informational requirements of the U.S. Securities Exchange Act of 1934 and, in accordance with this act, we file reports, including annual reports on Form 20-F, and other information with the SEC. You may read and copy any of this information in the SEC’s Public Reference Room, 100 F Street, N.E., Washington, D.C. 20549. You can obtain information on the operation of the SEC’s Public Reference Room in Washington, D.C. by calling the SEC at 1-800-SEC-0330.

The SEC also maintains an Internet web site that contains reports, proxy and information statements, and other information regarding issuers, like us, that file electronically with the SEC. The address of that site is http://www.sec.gov.

INCORPORATION OF DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate by reference” the information we file with or furnish to them. This means that we can disclose important information to you by referring you to those documents. Each document incorporated by reference is current only as of the date of such document, and the incorporation by reference of such documents shall not create any implication that there has been no change in our affairs since the date thereof or that the information contained therein is current as of any time subsequent to its date. The information incorporated by reference is considered to be a part of this prospectus supplement and should be read with the same care. When we update the information contained in documents that have been incorporated by reference by making future filings with the SEC, the information incorporated by reference in this prospectus supplement is considered to be automatically updated and superseded. In other words, in the case of a conflict or inconsistency between information contained in this prospectus supplement and information incorporated by reference into this prospectus supplement, you should rely on the information contained in the document that was filed later.

We incorporate by reference the documents listed below:

We will provide without charge to each person, including any beneficial owner, to whom this prospectus supplement and the accompanying prospectus are delivered, upon his or her written or oral request, a copy of any or all documents referred to above which have been or may be incorporated by reference into this prospectus supplement or the accompanying prospectus other than exhibits which are not specifically incorporated by reference in those documents. You can request those documents from:

CNOOC Limited

65th Floor, Bank of China Tower

One Garden Road, Central

Hong Kong

Tel +852 2213-2500

You should rely only on the information that we incorporate by reference or provide in this prospectus supplement or the accompanying prospectus. We have not authorized anyone to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it.

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PROSPECTUS

CNOOC Limited

Guarantees of Debt Securities

CNOOC Finance (2015) U.S.A. LLC

Debt Securities

We may from time to time offer in one or more series debt securities of CNOOC Finance (2015) U.S.A. LLC, which would be fully and unconditionally guaranteed by CNOOC Limited.

Each time we sell securities pursuant to this prospectus, we will provide a supplement to this prospectus that contains specific information about the offering and the specific terms of the securities offered. The applicable prospectus supplement may also add to, update or change information contained in this prospectus. In addition, we may supplement, update or change any of the information contained in this prospectus by incorporating information by reference in this prospectus.

You should read this prospectus, the applicable prospectus supplement and any documents incorporated by reference carefully before you invest in any of our securities. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any state or other jurisdiction where the offer or sale is not permitted.

Investing in our securities involves risk. You should carefully consider the risks described under the heading “Risk Factors” beginning on page 3, in the applicable prospectus supplement and in the documents incorporated by reference in this prospectus or the applicable prospectus supplement.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

Prospectus dated April 20, 2018

TABLE OF CONTENTS

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ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement we filed with the Securities and Exchange Commission (the “SEC”), using a “shelf” registration process. We may sell the securities described in this prospectus from time to time in one or more offerings. You should carefully read this prospectus and any prospectus supplement together with the additional information described under the heading “Where You Can Find More Information About Us.” We have not authorized anyone to provide you with different or additional information.

Each time we sell securities pursuant to this prospectus, we will provide a prospectus supplement that contains specific information about the terms of that offering, including the specific amounts, prices and terms of the securities offered. If this prospectus is inconsistent with the prospectus supplement, you should rely upon the prospectus supplement. In addition, the prospectus supplement may also add, update or change the information contained in this prospectus.

If you are in a jurisdiction where offers to sell, or solicitations of offers to purchase, the securities offered by this document are unlawful, or if you are a person to whom it is unlawful to direct these types of activities, then the offer presented in this document does not extend to you.

You should assume that the information in this prospectus or any prospectus supplement, as well as the information incorporated by reference in this prospectus or any prospectus supplement, is accurate only as of the date of the documents containing the information, unless the information specifically indicates that another date applies. Our business, financial condition, results of operations and prospects may have changed since those dates.

Wherever references are made in this prospectus to information that will be included in a prospectus supplement, to the extent permitted by applicable law, rules or regulations, we may instead include such information or add, update or change the information contained in this prospectus by means of a post-effective amendment to the registration statement of which this prospectus is a part, through filings we make with the SEC that are incorporated by reference in this prospectus or by any other method as may then be permitted under applicable law, rules or regulations.

In this prospectus, unless otherwise indicated, references to “we,” “us,” “our” and the “Company” refer to CNOOC Limited, or CNOOC Limited and its subsidiaries, including CNOOC Finance (2015) U.S.A. LLC (the “Issuer”), as the context requires. References to “CNOOC” are to China National Offshore Oil Corporation and its subsidiaries (other than CNOOC Limited and its subsidiaries). References to “China” and the “PRC” refer to the People’s Republic of China and, solely for the purpose of this prospectus, exclude the Hong Kong Special Administrative Region, the Macau Special Administrative Region and Taiwan.

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CNOOC LIMITED

We are an upstream company specializing in the exploration, development and production of oil and natural gas. We are the dominant oil and natural gas producer in offshore China and, in terms of reserves and production, we are also one of the largest independent oil and natural gas exploration and production companies in the world.

We were incorporated with limited liability on August 20, 1999 in Hong Kong under the Hong Kong Companies Ordinance. The PRC government established CNOOC, our controlling shareholder, as a state-owned offshore petroleum company in 1982 under the Regulation of the PRC on the Exploitation of Offshore Petroleum Resources in Cooperation with Foreign Enterprises. CNOOC assumed certain responsibility for the administration and development of PRC offshore petroleum operations with foreign oil and gas companies. Prior to CNOOC’s reorganization in 1999, CNOOC and its various affiliates performed both commercial and administrative functions relating to oil and natural gas exploration and development in offshore China. In 1999, CNOOC transferred all of its then current operational and commercial interests in its offshore petroleum business, including the related assets and liabilities, to us. As a result, we and our subsidiaries are the only vehicles through which CNOOC engages in oil and gas exploration, development, production and sales activities both in and outside the PRC.

Our registered office is located at 65th Floor, Bank of China Tower, One Garden Road, Central, Hong Kong, and our telephone number is +852 2213-2500. We maintain a website at www.cnoocltd.com where general information about us is available. The contents of the website are not part of this prospectus.

THE ISSUER

CNOOC Finance (2015) U.S.A. LLC is our indirect wholly-owned subsidiary and was formed as a limited liability company on March 23, 2015 in the State of Delaware. It has no material assets and will conduct no business except in connection with the issuance of debt securities and the advance of proceeds from such issuance to us or a company controlled by us. So long as the applicable series of debt securities are outstanding, CNOOC Finance (2015) U.S.A. LLC will limit its permitted activities as described under “Description of Debt Securities and Guarantees—Certain Covenants—Limitation on Issuer’s Activities.” Its registered office is located at the offices of its registered agent, Corporation Service Company, at 251 Little Falls Drive, Wilmington, Delaware, U.S.A. 19808. Its telephone number is 302-636-5400.

CNOOC Finance (2015) U.S.A. LLC is treated as a disregarded entity for U.S. federal income tax purposes.

2

RISK FACTORS

You should consider the specific risks described in our annual report on Form 20-F for the year ended December 31, 2017 under “Item 3. Key Information—Risk Factors,” the risks described under “Risk Factors” in the applicable prospectus supplement, and any risk factors set forth in our other filings with the SEC, pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, including our reports on Form 6-K, before making an investment decision. Each of the risks described in these documents could materially and adversely affect our business, financial condition, results of operations and prospects, and could result in a partial or complete loss of your investment. See “Where You Can Find More Information About Us.”

3

FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated by reference include “forward-looking statements” within the meaning of the United States Private Securities Litigation Reform Act of 1995, including statements regarding expected future events, business prospects or financial results. The words “expect,” “anticipate,” “continue,” “estimate,” “objective,” “ongoing,” “may,” “will,” “project,” “should,” “believe,” “plans,” “intends” and similar expressions are intended to identify such forward-looking statements.

These forward-looking statements address, among others, such issues as:

These statements are based on assumptions and analysis made by us in light of our experience and our perception of historical trends, current conditions and expected future developments, as well as other factors we believe are appropriate under the circumstances. However, whether actual results and developments will meet our expectations and predictions depend on a number of risks and uncertainties which could cause our actual results, performance and financial condition to differ materially from our expectations, including those associated with fluctuations in crude oil and natural gas prices, our exploration or development activities, our capital expenditure requirements, our business strategy, whether the transactions entered into by us can complete on schedule pursuant to their terms and timetable or at all, the highly competitive nature of the oil and natural gas industry, our foreign operations, environmental liabilities and compliance requirements, and economic and political conditions in the PRC and overseas. For a description of these and other risks and uncertainties, see “Risk Factors” and other cautionary statements appearing in this prospectus and the documents incorporated by reference.

Consequently, all of the forward-looking statements made in this prospectus and the documents incorporated by reference are qualified by these cautionary statements. We cannot assure that the results or developments anticipated by us will be realized or, even if substantially realized, that they will have the expected effect on us, our business or our operations.

4

WHERE YOU CAN FIND MORE INFORMATION ABOUT US

This prospectus is part of a registration statement that we filed with the SEC. The registration statement, including the attached exhibits, contains additional relevant information about us. The rules and regulations of the SEC allow us to omit some of the information included in the registration statement from this prospectus. In addition, we are subject to periodic reporting and other informational requirements of the Securities Exchange Act of 1934 and, in accordance with this act, we file reports, including annual reports on Form 20-F, and other information with the SEC. You may read and copy any of this information in the SEC’s Public Reference Room, 100 F Street, N.E., Washington, D.C. 20549. You can obtain information on the operation of the SEC’s Public Reference Room in Washington, D.C. by calling the SEC at 1-800-SEC-0330.

The SEC also maintains an Internet web site that contains reports, proxy and information statements, and other information regarding issuers, like us, that file electronically with the SEC. The address of that site is http://www.sec.gov.

INCORPORATION OF DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate by reference” the information we file with or furnish to them. This means that we can disclose important information to you by referring you to those documents. Each document incorporated by reference is current only as of the date of such document, and the incorporation by reference of such documents shall not create any implication that there has been no change in our affairs since the date thereof or that the information contained therein is current as of any time subsequent to its date. The information incorporated by reference is considered to be a part of this prospectus and should be read with the same care.

When we update the information contained in documents that have been incorporated by reference by making future filings with the SEC, the information incorporated by reference in this prospectus is considered to be automatically updated and superseded. In other words, in the case of a conflict or inconsistency between information contained in this prospectus and information incorporated by reference into this prospectus, you should rely on the information contained in the document that was filed later.

We incorporate by reference the documents listed below:

We will provide without charge to each person, including any beneficial owner, to whom this prospectus is delivered, upon his or her written or oral request, a copy of any or all documents referred to above which have been or may be incorporated by reference into this prospectus other than exhibits which are not specifically incorporated by reference into those documents. You can request those documents from:

CNOOC Limited

65th Floor, Bank of China Tower

One Garden Road, Central

Hong Kong

Tel +852 2213-2500

You should rely only on the information that we incorporate by reference or provide in this prospectus or the applicable prospectus supplement. We have not authorized anyone to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it.

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RATIO OF EARNINGS TO FIXED CHARGES

The following table sets forth our unaudited consolidated ratio of earnings to fixed charges for each of the periods indicated using financial information extracted, where applicable, from our audited consolidated financial statements. Our audited consolidated financial statements are prepared in accordance with International Financial Reporting Standards as issued by the International Accounting Standards Board (“IFRS IASB”).

Earnings included in the calculation of the ratio of earnings to fixed charges represent income before income taxes plus fixed charges, other than capitalized interest. Fixed charges include interest expense, including capitalized interest, amortization of debt issuance costs and a portion of rent expense representative of interest.

USE OF PROCEEDS

Unless otherwise indicated in an accompanying prospectus supplement, the net proceeds from the sale of securities will be used for general corporate purposes.

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DESCRIPTION OF DEBT SECURITIES AND GUARANTEES

We may from time to time offer in one or more series debt securities of the Issuer which would be fully and unconditionally guaranteed by the Company. The following is a description of certain general terms and provisions of the debt securities, the guarantees and an indenture (the “indenture”) entered into or to be entered into among the Issuer, the Company and The Bank of New York Mellon as trustee (the “trustee”), initial paying and transfer agent and initial registrar, the form of which has been filed as an exhibit to the registration statement of which this prospectus is a part.

This description is not complete and is subject to and qualified in its entirety by reference to all of the provisions of the indenture, including the definitions of specified terms used in the indenture, and to the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”). The particular terms of the debt securities to be offered by any prospectus supplement and the extent these general provisions may apply to such debt securities will be described in the applicable prospectus supplement. The terms of such debt securities will include those set forth in the indenture and any related documents and those made a part of the indenture by the Trust Indenture Act. You should read the description below, the applicable prospectus supplement and the provisions of the indenture and any related documents before investing in the debt securities.

General

The prospectus supplement relating to any series of debt securities that we may offer will contain the specific terms of the debt securities. These terms may include the following:

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We may sell the debt securities, including original issue discount securities, at par or at greater than de minimis discount below their stated principal amount. Unless otherwise stated in a prospectus supplement, additional debt securities of a particular series may be issued without the consent of the holders of the debt securities of such series outstanding at the time of issuance. Any such additional debt securities, together with all other outstanding debt securities of that series, will constitute a single series of debt securities under the indenture. Such additional debt securities will have the same terms and conditions as the applicable series of debt securities in all respects (or in all respects except for the issue date, the issue price or the first payment of interest), and will vote together as one class on all matters with respect to such series of debt securities. No additional debt securities will be issued with the same CUSIP, ISIN or other identifying number as the debt securities of a series issued hereunder unless the additional debt securities are fungible with such outstanding debt securities for U.S. federal income tax purposes.

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Form, Exchange and Transfer

The debt securities will be issued, unless otherwise indicated in the applicable prospectus supplement, in fully registered form without interest coupons and in minimum denominations of US$2,000 and integral multiples of US$1,000 in excess thereof.

The entity performing the role of maintaining the list of registered holders is called the “registrar.” It will also register transfers of the registered debt securities. You may exchange or transfer your registered debt securities at the office of the registrar. The registrar acts as the agent of the Issuer for registering debt securities in the names of holders and transferring registered debt securities. The Issuer may also arrange for additional registrars, and may change registrars. The Issuer may also choose to act as its own registrar.

You will not be required to pay a service charge for any registration of transfer or exchange of debt securities, but you may be required to pay any tax or other governmental charge associated with the registration of transfer or exchange. The registration of transfer or exchange of a registered debt security will only be made if you have duly endorsed the debt security or provided the registrar with a written instrument of transfer satisfactory in form to the registrar.

Guarantees

Under the indenture, the Company will irrevocably and unconditionally guarantee the due and punctual payment of the principal of and interest on, and all other amounts payable under (including any Additional Amounts payable in respect of), the debt securities when and as the same shall become due and payable, whether on the stated maturity, upon acceleration, by call for redemption or otherwise. The Company has (i) agreed that its obligations under the guarantees will be as if it were principal obligor and not merely surety, and will be enforceable irrespective of any invalidity, irregularity or unenforceability of the debt securities or the indenture (other than in respect of the guarantees) and (ii) waived its right to require the trustee to pursue or exhaust its legal or equitable remedies against the Issuer prior to exercising the trustee’s rights under the guarantees. The guarantees will not be discharged with respect to any debt security except by payment in full of the principal thereof, interest thereon and all other amounts payable thereunder (including any Additional Amounts payable in respect thereof). Moreover, if at any time any amount paid under a debt security is rescinded or must otherwise be restored, the rights of the holder of the debt security under the guarantee will be reinstated with respect to such payment as though such payment had not been made. All payments under the guarantees will be made in U.S. dollars.

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Additional Amounts

All payments of principal and interest in respect of the debt securities of each series and/or the guarantees will be made free and clear of, and without withholding or deduction for, or on account of, any present or future taxes, duties, assessments or governmental charges of whatever nature (“Taxes”) imposed, levied, collected, withheld or assessed by or on behalf of the United States, Hong Kong, the PRC or any other jurisdiction in which the Issuer or the Company (or any successor to the Issuer or the Company) is resident for tax purposes, in each case including any political subdivision, territory or possession thereof, any authority therein having power to tax or any area subject to its jurisdiction, or any jurisdiction from or through which any payment is made by or on behalf of the Issuer or the Company (each, a “Relevant Taxing Jurisdiction”) unless such Taxes are required by law to be withheld or deducted. If any deduction or withholding for any present or future Taxes of the applicable Relevant Taxing Jurisdiction shall at any time be so required, the Issuer or the Company, as the case may be, shall pay such additional amounts (“Additional Amounts”) as will result (after deduction of such Taxes and any additional Taxes payable in respect of such Additional Amounts) in receipt by the holder of any debt security of such amounts as would have been received by such holder with respect to such security or the guarantee, as applicable, had no such withholding or deduction been required; provided, however, that no Additional Amounts shall be payable in respect of any debt security:

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Additional Amounts will also not be paid with respect to any payment of the principal of or any interest on any debt security or under the guarantee to any holder of a debt security who is a fiduciary or partnership or other than the sole beneficial owner of such payment to the extent that payment would be required by the laws of the Relevant Taxing Jurisdiction to be included in the income of a beneficiary or settlor with respect to the fiduciary, a member of that partnership or a beneficial owner who would not have been entitled to the Additional Amounts had that beneficiary, settlor, member or beneficial owner been the holder of the debt securities.

Whenever there is mentioned, in any context, the payment of principal or interest in respect of any debt security or guarantee, such mention shall be deemed to include the payment of Additional Amounts provided for in the indenture to the extent that, in such context, Additional Amounts are, were or would be payable in respect thereof pursuant to the indenture.

Optional Tax Redemption

Each series of debt securities may be redeemed, at the option of the Issuer, in whole but not in part, upon notice as described below, at a redemption price equal to 100% of the principal amount thereof, together with accrued interest to, but not including, the date fixed for redemption and Additional Amounts, if any, if, as a result of any change in or amendment to the laws of a Relevant Taxing Jurisdiction with respect to the Company or such Issuer or any regulations or rulings promulgated thereunder, or any change in the official interpretation or official application of such laws, regulations or rulings, which change or amendment (i) in the case of the Company or the Issuer becomes effective on or after the date of the applicable prospectus supplement and (ii) in the case of any successor to the Company or the Issuer that is organized or tax resident in a jurisdiction that is not a Relevant Taxing Jurisdiction with respect to the Company or such Issuer as of the original issue date of the applicable series of debt securities becomes effective on or after the date such successor assumes the Company’s or the Issuer’s obligations, as applicable, under the debt securities and the indenture,

11

and such obligation cannot be avoided by the use of reasonable measures available to the Company or the Issuer, as the case may be.