UNDERWRITING AGREEMENT

EXHIBIT 1.1

OCCIDENTAL PETROLEUM CORPORATION

UNDERWRITING AGREEMENT

May 12, 2009

Barclays Capital Inc.

745 Seventh Avenue

New York, New York 10019

Citigroup Global Markets Inc.

388 Greenwich Street

New York, New York 10013

J.P. Morgan Securities Inc.

270 Park Avenue

New York, New York 10017

UBS Securities LLC

677 Washington Boulevard

Stamford, Connecticut 06901

as Representatives of the several Underwriters

Ladies and Gentlemen:

Occidental Petroleum Corporation, a Delaware corporation (the “Company”), confirms its agreement with the underwriters listed on Schedule A hereto (collectively the “Underwriters,” which term shall also include any underwriter substituted as hereinafter provided in Section 11 hereof) with respect to the issue and sale by the Company of $750,000,000 aggregate principal amount of the Company’s 4.125% Senior Notes due 2016 (the “Notes”) and the purchase by the Underwriters, acting severally and not jointly, of the respective principal amounts of Notes set forth opposite their names on Schedule A hereto. The Notes are to be issued pursuant to an indenture, dated as of April 1, 1998 (the “Indenture,” which term, for purposes of this Agreement, shall include an Officers’ Certificate or supplemental indenture with respect to the Notes delivered pursuant to Section 301 of the Indenture), between the Company and The Bank of New York Mellon Trust Company, N.A., as successor to The Bank of New York, as trustee (the “Trustee”). Barclays Capital Inc., Citigroup Global Markets Inc., J.P. Morgan Securities Inc., and UBS Securities LLC shall be the representatives of the Underwriters (the “Representatives;” in the event that there is only one such representative, then all references herein to the “Representatives” shall be deemed to mean and refer to such single representative, mutatis mutandis).

1

The Company has filed with the Securities and Exchange Commission (the “Commission”) a registration statement on Form S-3 (No. 333-152875) and Post-Effective Amendment No. 1 thereto (“Post-Effective Amendment No. 1”) for the registration of debt securities, including the Notes, under the Securities Act of 1933, as amended (the “1933 Act”), and the offering thereof from time to time in accordance with Rule 415 of the rules and regulations of the Commission under the 1933 Act (the “1933 Act Regulations”). Such registration statement and Post-Effective Amendment No. 1 became effective upon filing with the Commission pursuant to Rule 462(e) of the 1933 Act Regulations, and the Indenture has been qualified under the Trust Indenture Act of 1939, as amended (the “1939 Act”). Such registration statement (as amended by Post-Effective Amendment No. 1 and any subsequent post-effective amendments thereto) and the prospectus dated August 11, 2008 (the “Base Prospectus”) together with the final prospectus supplement dated May 12, 2009 (the “Final Prospectus Supplement”) relating to the Notes, including, in each case, all Incorporated Documents (as hereinafter defined) and, solely in the case of any such registration statement, the information that is deemed pursuant to Rule 430B of the 1933 Act Regulations to be part of such registration statement (“Rule 430B Information”), are referred to herein as the “Registration Statement” and the “Prospectus,” respectively, except that, if any revised prospectus or prospectus supplement filed by the Company shall be provided to the Underwriters by the Company for use in connection with the offering of the Notes (including, without limitation, for delivery upon request of purchasers of Notes pursuant to Rule 173 of the 1933 Act Regulations), the term “Prospectus” shall refer to such revised prospectus or prospectus supplement, as the case may be, from and after the time it is first provided to the Underwriters for such use. As used herein, the term “preliminary prospectus” means any prospectus supplement filed by the Company relating to the Notes that is captioned “Subject to Completion” or “preliminary prospectus supplement” or that has a similar caption, together with the Base Prospectus, including all Incorporated Documents, it being understood that all references herein to a “preliminary prospectus” shall include, without limitation, the Statutory Prospectus (as defined below). Any reference herein to the Registration Statement, any preliminary prospectus or the Prospectus shall be deemed to refer to and include the documents, financial statements and schedules incorporated, or deemed to be incorporated, by reference therein (other than information in such documents, financial statements and schedules that is deemed not to be filed) pursuant to Item 12 of Form S-3 under the 1933 Act, and any reference to any amendment or supplement to the Registration Statement, any preliminary prospectus or the Prospectus shall be deemed to refer to and include any documents, financial statements and schedules filed by the Company with the Commission under the Securities Exchange Act of 1934, as amended (the “1934 Act”), and so incorporated, or deemed to be incorporated, by reference (other than information in such documents, financial statements and schedules that is deemed not to be filed) (such incorporated documents, financial statements and schedules being herein called the “Incorporated Documents”). Notwithstanding the foregoing, for purposes of this Agreement any prospectus supplement prepared or filed with respect to an offering pursuant to the Registration Statement of any securities other than the Notes shall not be deemed to have supplemented any preliminary prospectus or the Prospectus and the information therein shall not be deemed Rule 430B Information. For purposes of this Agreement, all references to the Registration Statement, the Prospectus or any preliminary prospectus, or to any Issuer Free Writing Prospectus, Issuer General Use Free Writing Prospectus or Issuer Limited Use Free Writing Prospectus (as such terms are hereinafter defined) or to any amendment or supplement to any of the foregoing shall be deemed to include any copy

2

filed with the Commission pursuant to its Electronic Data Gathering, Analysis and Retrieval system or its Interactive Data Electronic Applications system (collectively, “EDGAR”).

The Company understands that the Underwriters propose to make a public offering of the Notes as soon as the Underwriters deem advisable after this Agreement has been executed and delivered.

SECTION 1.    Representations and Warranties.

(a)   The Company represents and warrants to each of the Underwriters as of the date hereof, as of the Applicable Time (as defined below) and as of the Closing Time referred to in Section 2(b) hereof, as follows:

(i)  (A) At the respective times of filing the Registration Statement and any post-effective amendments thereto, (B) at the time of the most recent amendment thereto for the purposes of complying with Section 10(a)(3) of the 1933 Act (whether such amendment was by post-effective amendment, incorporated report filed pursuant to Section 13 or 15(d) of the 1934 Act or form of prospectus) and (C) at the time the Company or any person acting on its behalf (within the meaning, for this clause only, of Rule 163(c) of the 1933 Act Regulations) made any offer relating to the Notes in reliance on the exemption of Rule 163 of the 1933 Act Regulations, the Company was or is (as the case may be) a “well-known seasoned issuer,” as defined in Rule 405 of the 1933 Act Regulations (“Rule 405”), including not having been and not being an “ineligible issuer” as defined in Rule 405; the Registration Statement is an “automatic shelf registration statement,” as defined in Rule 405, that initially became effective within three years of the date hereof; and the Company has not received from the Commission any notice pursuant to Rule 401(g)(2) of the 1933 Act Regulations objecting to the use of the automatic shelf registration statement form.

At the earliest time after the filing of the Registration Statement that the Company or another offering participant made a bona fide offer (within the meaning of Rule 164(h)(2) of the 1933 Act Regulations) of the Notes, the Company was not and is not an “ineligible issuer,” as defined in Rule 405, without taking account of any determination by the Commission pursuant to Rule 405 that it is not necessary that the Company be considered an “ineligible issuer.”

(ii)  The Incorporated Documents filed with the Commission subsequent to December 31, 2008, when they were filed (or, if an amendment with respect to any such Incorporated Document was filed, when such amendment was filed) with the Commission, as the case may be, complied in all material respects with the requirements of the 1934 Act and the rules and regulations of the Commission under the 1934 Act (the “1934 Act Regulations”), and any Incorporated Documents filed subsequent to the date hereof and prior to the termination of the offering of the Notes will, when they are filed with the Commission, comply in all material respects with the requirements of the 1934 Act and the 1934 Act Regulations; no such Incorporated Document, when it was filed (or, if an amendment with respect to any such Incorporated Document was filed, when such amendment was filed) with the Commission, contained, and no Incorporated Document

3

filed subsequent to the date hereof and prior to the termination of the offering of the Notes will contain, when read together with the other information in the Registration Statement, the General Disclosure Package or the Prospectus, an untrue statement of a material fact or omitted, or will omit, to state a material fact required to be stated therein or necessary to make the statements therein, in the light of the circumstances under which they were or will be made, not misleading.

(iii)  The Registration Statement, at the respective times Post-Effective Amendment No. 1 or any subsequent post-effective amendment thereto first became effective, at the time that the Company’s Annual Report on Form 10-K for the year ended December 31, 2008 or any amendment thereto was filed with the Commission, and at each deemed effective date with respect to the Underwriters pursuant to Rule 430B(f)(2) of the 1933 Act Regulations, complied and will comply in all material respects with the provisions of the 1933 Act and the 1933 Act Regulations and did not and will not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading, and the Prospectus and any supplement or amendment thereto relating to the Notes, as of their respective dates, at each deemed effective date with respect to the Underwriters pursuant to Rule 430B(f)(2) of the 1933 Act regulations, and as of the Closing Date, complied and will comply in all material respects with the provisions of the 1933 Act and the 1933 Act Regulations and did not and will not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary in order to make the statements therein, in the light of the circumstances under which they were or will be made, not misleading.

As of the Applicable Time, neither (x) the Final Term Sheet (as defined below), any other Issuer General Use Free Writing Prospectus(es) issued at or prior to the Applicable Time and the Statutory Prospectus, all considered together (collectively, the “General Disclosure Package”), nor (y) any individual Issuer Limited Use Free Writing Prospectus issued at or prior to the Applicable Time, when considered together with the General Disclosure Package, included any untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

Each Issuer Free Writing Prospectus, as of its issue date and at all subsequent times through the end of the period of its use, did not, does not and will not include any information that conflicted, conflicts or will conflict with the information contained in the Registration Statement or the Prospectus, including any document incorporated by reference therein and any preliminary or other prospectus deemed to be a part thereof that has not been superseded or modified.

As used in this subsection and elsewhere in this Agreement:

“Applicable Time” means 3:39 pm (New York City time) on May 12, 2009 or such other time as agreed by the Company and the Representatives.

4

“Issuer Free Writing Prospectus” means any “issuer free writing prospectus,” as defined in Rule 433 of the 1933 Act Regulations (“Rule 433”), relating to the Notes that (i) is required to be filed with the Commission by the Company, (ii) is a “road show” that constitutes a written communication within the meaning of Rule 433(d)(8)(i), whether or not required to be filed with the Commission or (iii) is exempt from filing pursuant to Rule 433(d)(5)(i) because it contains a description of the Notes or of the offering thereof that does not reflect the final terms, in each case, in the form filed or required to be filed with the Commission or, if not required to be filed, in the form retained in the Company’s records pursuant to Rule 433(g).

“Issuer General Use Free Writing Prospectus” means any Issuer Free Writing Prospectus that is intended for general distribution to prospective investors, as evidenced by it being specified in Schedule C hereto.

“Issuer Limited Use Free Writing Prospectus” means any Issuer Free Writing Prospectus that is not an Issuer General Use Free Writing Prospectus.

“Statutory Prospectus” means the Base Prospectus and the preliminary prospectus supplement dated May 12, 2009 relating to the Notes, including the Incorporated Documents.

The foregoing representations and warranties in this subsection do not apply to statements or omissions in the Registration Statement, any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus, or any amendments or supplements thereto, made in reliance upon and in conformity with information furnished to the Company in writing by or on behalf of the Underwriters through the Representatives expressly for use therein or to those parts of the Registration Statement which constitute the Trustee’s Statements of Eligibility and Qualification on Form T-1 under the 1939 Act (collectively, the “Form T-1”).

There is no contract or document of a character required to be described in the Registration Statement, the Statutory Prospectus or the Prospectus or to be filed as an exhibit to the Registration Statement that is not described or filed as required.

(iv)  This Agreement, the Indenture and the Notes have been duly authorized by the Company and, to the extent described in the General Disclosure Package or the Prospectus, are fairly and accurately summarized therein in all material respects.

(v)  The Indenture has been duly qualified under the 1939 Act and duly executed and delivered by the Company and (assuming the due execution and delivery thereof by the Trustee) is, and the Notes (when issued by the Company and authenticated in accordance with the Indenture and delivered to and paid for by the Underwriters) will have been duly executed or delivered by the Company and will be, the legal, valid and binding obligations of the Company, enforceable against the Company in accordance with their terms, except (x) as such enforceability may be subject to or limited by (A) bankruptcy, insolvency, reorganization, moratorium or other similar laws now or hereafter in effect relating to or affecting the enforcement of creditors’ rights generally,

5

(B) the applicability or effect of any fraudulent transfer, preference or similar law, (C) general principles of equity (regardless of whether such enforcement is considered in a proceeding in equity or at law) or (D) the effect of general rules of contract law that limit the enforceability of provisions requiring indemnification of a party for liability for its own action or inaction to the extent the action or inaction involves gross negligence, recklessness, willful misconduct or unlawful conduct and (y) that the waiver contained in Section 515 of the Indenture may be deemed unenforceable. The Notes (when issued by the Company and authenticated in accordance with the terms of the Indenture and delivered to and paid for by the Underwriters) will be entitled to the benefits of the Indenture (subject to the exceptions set forth in the preceding sentence).

(vi)  The Company and each of Occidental Chemical Holding Corporation, a California corporation, and Occidental Oil and Gas Holding Corporation, a California corporation formerly known as Occidental Oil and Gas Corporation (each a “Principal Domestic Subsidiary” and collectively the “Principal Domestic Subsidiaries”) is a validly existing corporation in good standing under the laws of its state of incorporation. The Company and each Principal Domestic Subsidiary has requisite corporate power and corporate authority to own its respective properties and carry on its respective business as presently conducted, as described in the General Disclosure Package and the Prospectus, and is duly registered or qualified to conduct business, and is in good standing, in each jurisdiction in which it owns or leases property or transacts business and in which such registration or qualification is necessary, except as to jurisdictions where the failure to do so would not have a material adverse effect on the Company and its subsidiaries, taken as a whole. All of the outstanding capital stock or other securities evidencing equity ownership of each Principal Domestic Subsidiary has been duly authorized and validly issued and is fully paid and non-assessable, and, except as otherwise disclosed in the Registration Statement, the General Disclosure Package and the Prospectus, is owned by the Company, directly or indirectly through subsidiaries, free and clear of any security interest, claim, lien or encumbrance.

(vii)  Except as contemplated in the General Disclosure Package and the Prospectus or reflected therein by the filing of any amendment or supplement thereto or any Incorporated Document, since the date of the most recent consolidated financial statements included or incorporated by reference in the Registration Statement, the General Disclosure Package and the Prospectus, unless the Company has notified the Underwriters as provided in Section 4(d) hereof, there has not been any material adverse change, or any development which is reasonably likely to result in a material adverse change, in the consolidated financial condition or consolidated results of operations of the Company and its subsidiaries, taken as a whole.

(viii)  The Company is not in violation of its Restated Certificate of Incorporation or Bylaws, in each case, as amended. The execution and delivery of this Agreement by the Company, the issuance and sale of the Notes and the performance by the Company of its obligations under this Agreement and the Indenture do not and will not violate or constitute a breach of or a default (with the passage of time or otherwise) under (A) the Restated Certificate of Incorporation or Bylaws of the Company, in each case, as amended, (B) any agreement or instrument (which is, individually or in the

6

aggregate, material to the Company and its subsidiaries, taken as a whole) to which the Company or any Principal Domestic Subsidiary is a party or by which any of them is bound or to which any of the property or assets of the Company or any Principal Domestic Subsidiary is subject or (C) any order of any court or governmental agency or authority (which is, individually or in the aggregate, material to the Company and its subsidiaries, taken as a whole) presently in effect and applicable to the Company or any Principal Domestic Subsidiary. Except for orders, permits and similar authorizations required under the securities or Blue Sky laws of certain jurisdictions, including jurisdictions outside the United States, or required of any securities exchange on which any of the Notes might be listed, no consent, approval, authorization or other order of any regulatory body, administrative agency or other governmental body is legally required for the valid issuance and sale of the Notes.

(ix)  To the best of the Company’s knowledge, the accountants who have audited and reported upon the consolidated financial statements filed with the Commission as part of the Registration Statement, the General Disclosure Package and the Prospectus are independent registered public accountants as required by the 1933 Act. The consolidated financial statements included in the Registration Statement, the General Disclosure Package and the Prospectus, or incorporated therein by reference, fairly present in all material respects the consolidated financial position and results of operations of the entities to which such statements relate at the respective dates and for the respective periods to which they apply. Such consolidated financial statements have been prepared in accordance with generally accepted accounting principles consistently applied, except as set forth in the Registration Statement, the General Disclosure Package and Prospectus.

(x)  The Company maintains “disclosure controls and procedures” (as defined in Rule 13a-15(e) of the 1934 Act) that are designed to ensure that information required to be disclosed by the Company in reports that it files or submits under the 1934 Act is recorded, processed, summarized and reported within the time periods specified in the Commission’s rules and forms, including controls and procedures designed to ensure that such information is accumulated and communicated to the Company’s management as appropriate to allow timely decisions regarding required disclosure. The Company has carried out evaluations of the effectiveness of their disclosure controls and procedures as required by Rule 13a-15 of the 1934 Act.

(xi)  The Company maintains “internal control over financial reporting” (as defined in Rule 13a-15(f) of the 1934 Act) that have been designed by, or under the supervision of, its principal executive and principal financial officers, or persons performing similar functions, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles and that include those policies and procedures that (i) pertain to the maintenance of records that in reasonable detail accurately and fairly reflect the transactions and dispositions of the assets of the Company; (ii) provide reasonable assurance that transactions are recorded as necessary to permit preparation of financial statements in accordance with generally accepted accounting principles and that receipts and expenditures of the Company are being made

7

only in accordance with the authorizations of management and the directors of the Company; and (iii) provide reasonable assurance regarding prevention or timely detection of unauthorized acquisition, use or disposition of the Company’s assets that could have a material adverse effect on the Company’s financial statements.

(b)    Additional Certifications . Any certificate signed by any officer of the Company and delivered to the Underwriters, the Representatives or to counsel for the Underwriters in connection with transactions contemplated hereby shall be deemed a representation and warranty by the Company to the Underwriters as to the matters covered thereby on the date of such certificate.

SECTION 2.    Sale and Delivery to the Underwriters; Closing.

(a)    Purchase and Sale . On the basis of the representations and warranties herein contained and subject to the terms and conditions herein set forth, the Company agrees to issue and sell to each Underwriter, severally and not jointly, and each Underwriter, severally and not jointly, agrees to purchase from the Company, at 98.667% of the principal amount thereof, the principal amount of Notes set forth on Schedule A opposite the name of such Underwriter.

(b)    Closing Time . Payment of the purchase price for, and delivery of the Notes shall be made at the offices of Sidley Austin LLP, 555 California Street, 20 ^th Floor, San Francisco, California 94104, or at such other place as shall be agreed upon by the Underwriters and the Company, at 10:00 a.m., New York City time, on May 15, 2009 (unless postponed in accordance with the provisions of Section 11), or such other time not later than ten business days after such date as shall be agreed upon by the Representatives and the Company (such time and date of payment and delivery being herein called “Closing Time”). Payment shall be made to the Company by wire transfer of immediately available funds to a bank account designated by the Company, against delivery to the nominee of The Depository Trust Company, for the account of the Underwriters, of one or more global notes representing the Notes (the “Global Notes”) to be purchased by them. It is understood that each Underwriter has authorized the Representatives for its respective account, to accept delivery of, and receipt for, and make payment of the purchase price for, the Notes which such Underwriter has agreed to purchase. The Global Notes will be made available for examination and packaging by the Underwriters not later than 10:00 a.m. New York City time on the last business day prior to Closing Time in The City of New York.

(c)    No Fiduciary Relationship . The Company acknowledges and agrees that the Underwriters are acting solely in the capacity of an arm’s length contractual counterparty to the Company with respect to the offering of Notes contemplated hereby (including in connection with determining the terms of the offering) and not as a financial advisor or a fiduciary to, or an agent of, the Company or any other person. Additionally, neither the Representatives nor any other Underwriter is advising the Company or any other person as to any legal, tax, investment, accounting or regulatory matters in any jurisdiction. The Company shall consult with its own advisors concerning such matters and shall be responsible for making its own independent investigation and appraisal of the transactions contemplated hereby, and the Underwriters shall have no responsibility or liability to the Company with respect thereto. Any review by the Underwriters of the Company, the transactions contemplated hereby or any other matters relating

8

to such transactions will be performed solely for the benefit of the Underwriters and shall not be on behalf of the Company.

SECTION 3.    Foreign Offerings.

Each Underwriter, severally and not jointly, represents and agrees that (i) it has not solicited, and will not solicit, offers to purchase any of the Notes from, (ii) it has not sold, and will not sell, any of the Notes to, and (iii) it has not distributed, and will not distribute, the General Disclosure Package or the Prospectus to, any person or entity in any jurisdiction outside of the United States (collectively “Foreign Offers and Sales”) except, in each case, in compliance in all material respects with all applicable laws and, in connection with the initial offering of, or subscription for, any of the Notes, in full compliance with the requirements and procedures, if any, established by the Company, in a writing delivered to the Representatives, with respect to any such Foreign Offers and Sales. For the purposes of this paragraph, “United States” means the United States of America, its territories, its possessions (including the Commonwealth of Puerto Rico) and other areas subject to its jurisdiction.

In particular and without limiting the generality of the foregoing:

(i)  Each Underwriter, severally and not jointly, agrees to distribute, in connection with any Foreign Offers and Sales, only those Prospectuses used in connection therewith that have been appropriately “stickered” for use in the jurisdiction in which such Foreign Offers and Sales are to be made.

(ii)  With respect to the United Kingdom, each Underwriter represents and agrees, severally and not jointly, that: (A) it has only communicated or caused to be communicated and will only communicate or cause to be communicated an invitation or inducement to engage in investment activity (within the meaning of Section 21 of the Financial Services and Markets Act 2000, as amended (“FSMA”)) received by it in connection with the issue or sale of the Notes in circumstances in which Section 21(1) of the FSMA would not apply to the Company; and (B) it has complied and will comply with all applicable provisions of FSMA with respect to anything done by it in relation to the Notes in, from or otherwise involving the United Kingdom.

(iii)  In relation to each Member State of the European Economic Area which has implemented the Prospectus Directive (each, a “Relevant Member State”), each Underwriter represents and agrees, severally and not jointly, that with effect from and including the date on which the Prospectus Directive is implemented in that Relevant Member State (the “Relevant Implementation Date”) it has not made and will not make an offer of Notes to the public in that Relevant Member State prior to the publication of a prospectus in relation to the Notes which has been approved by the competent authority in that Relevant Member State or, where appropriate, approved in another Relevant Member State and notified to the competent authority in that Relevant Member State, all in accordance with the Prospectus Directive, except that it may, with effect from and including the Relevant Implementation Date, make an offer of Notes to the public in that Relevant Member State at any time:

9

For the purposes of this subparagraph (iii), the expressio