SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  Schedule 13D

                    Under the Securities Exchange Act of 1934

                                 Amendment No. 1


                                  Premcor Inc.
          ------------------------------------------------------------
                                (Name of Issuer)


                                  Common Stock
          ------------------------------------------------------------
                         (Title of Class of Securities)


                                   74045Q 10 4
          ------------------------------------------------------------
                                 (CUSIP Number)


                            Donald P. de Brier, Esq.
                        Occidental Petroleum Corporation
                            10889 Wilshire Boulevard
                          Los Angeles, California 90024
                                 (310) 208-8800
          ------------------------------------------------------------
            (Name, Address and Telephone Number of Person Authorized
                     to Receive Notices and Communications)


                                January 29, 2003
          ------------------------------------------------------------
             (Date of Event Which Requires Filing of this Statement)


If the filing person has previously filed a statement on Schedule 13G to report
the acquisition which is the subject of this Schedule 13D, and is filing this
schedule because of Rule 13d-l(e), 13d-1(f) or 13d-1(g), check the following box
[ ]


                               Page 1 of 18 Pages

CUSIP No. 74045Q 10 4 ----------- (1) Names of reporting persons. Occidental Petroleum Corporation I.R.S. Identification Nos. of above persons (entities only). 95-4035997 (2) Check the appropriate box if a member of a group (a) (see instructions) (b) (3) SEC use only. (4) Source of funds (see instructions). OO (5) Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e). (6) Citizenship or place of organization. Delaware Number of shares beneficially owned by each reporting person with: (7) Sole voting power. 1,908,209 (8) Shared voting power. 7,126,437 (9) Sole dispositive power. 1,908,209 (10) Shared dispositive power. 7,126,437 (11) Aggregate amount beneficially owned by each reporting person 9,034,646 (12) Check if the aggregate amount in Row (11) excludes certain shares (see instructions). (13) Percent of class represented by amount in Row (11). 12.31% (14) Type of reporting person (see instructions). CO Page 2 of 18 Pages

CUSIP No. 74045Q 10 4 ----------- (1) Names of reporting persons. Occidental C.O.B. Partners I.R.S. Identification Nos. of above persons (entities only). 73-1484588 (2) Check the appropriate box if a member of a group (a) (see instructions) (b) (3) SEC use only. (4) Source of funds (see instructions). OO (5) Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e). (6) Citizenship or place of organization. Delaware Number of shares beneficially owned by each reporting person with: (7) Sole voting power. (8) Shared voting power. 7,126,437 (9) Sole dispositive power. (10) Shared dispositive power. 7,126,437 (11) Aggregate amount beneficially owned by each reporting person 7,126,437 (12) Check if the aggregate amount in Row (11) excludes certain shares (see instructions). (13) Percent of class represented by amount in Row (11). 9.71% (14) Type of reporting person (see instructions). PN Page 3 of 18 Pages

CUSIP No. 74045Q 10 4 ----------- (1) Names of reporting persons. Placid Oil Company I.R.S. Identification Nos. of above persons (entities only). 72-0395174 (2) Check the appropriate box if a member of a group (a) (see instructions) (b) (3) SEC use only. (4) Source of funds (see instructions). OO (5) Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e). (6) Citizenship or place of organization. Delaware Number of shares beneficially owned by each reporting person with: (7) Sole voting power. (8) Shared voting power. 7,126,437 (9) Sole dispositive power. (10) Shared dispositive power. 7,126,437 (11) Aggregate amount beneficially owned by each reporting person 7,126,437 (12) Check if the aggregate amount in Row (11) excludes certain shares (see instructions). (13) Percent of class represented by amount in Row (11). 9.71% (14) Type of reporting person (see instructions). CO Page 4 of 18 Pages

CUSIP No. 74045Q 10 4 ----------- (1) Names of reporting persons. OXY USA Inc. I.R.S. Identification Nos. of above persons (entities only). 73-1166880 (2) Check the appropriate box if a member of a group (a) (see instructions) (b) (3) SEC use only. (4) Source of funds (see instructions). OO (5) Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e). (6) Citizenship or place of organization. Delaware Number of shares beneficially owned by each reporting person with: (7) Sole voting power. (8) Shared voting power. 7,126,437 (9) Sole dispositive power. (10) Shared dispositive power. 7,126,437 (11) Aggregate amount beneficially owned by each reporting person 7,126,437 (12) Check if the aggregate amount in Row (11) excludes certain shares (see instructions). (13) Percent of class represented by amount in Row (11). 9.71% (14) Type of reporting person (see instructions). CO Page 5 of 18 Pages

CUSIP No. 74045Q 10 4 ----------- (1) Names of reporting persons. Occidental Oil and Gas Holding Corporation I.R.S. Identification Nos. of above persons (entities only). 95-2864974 (2) Check the appropriate box if a member of a group (a) (see instructions) (b) (3) SEC use only. (4) Source of funds (see instructions). OO (5) Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e). (6) Citizenship or place of organization. California Number of shares beneficially owned by each reporting person with: (7) Sole voting power. (8) Shared voting power. 7,126,437 (9) Sole dispositive power. (10) Shared dispositive power. 7,126,437 (11) Aggregate amount beneficially owned by each reporting person 7,126,437 (12) Check if the aggregate amount in Row (11) excludes certain shares (see instructions). (13) Percent of class represented by amount in Row (11). 9.71% (14) Type of reporting person (see instructions). CO Page 6 of 18 Pages

CUSIP No. 74045Q 10 4 ----------- (1) Names of reporting persons. Occidental Petroleum Investment Co. I.R.S. Identification Nos. of above persons (entities only). 95-2584267 (2) Check the appropriate box if a member of a group (a) (see instructions) (b) (3) SEC use only. (4) Source of funds (see instructions). OO (5) Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e). (6) Citizenship or place of organization. California Number of shares beneficially owned by each reporting person with: (7) Sole voting power. (8) Shared voting power. 7,126,437 (9) Sole dispositive power. (10) Shared dispositive power. 7,126,437 (11) Aggregate amount beneficially owned by each reporting person 7,126,437 (12) Check if the aggregate amount in Row (11) excludes certain shares (see instructions). (13) Percent of class represented by amount in Row (11). 9.71% (14) Type of reporting person (see instructions). CO Page 7 of 18 Pages

ITEM 1. SECURITY AND ISSUER. Title of class of equity security: Common Stock Issuer: Premcor Inc. Address of principal executive offices: 1700 East Putnam Street, Suite 500 Old Greenwich, Connecticut 06870 ITEM 2. IDENTITY AND BACKGROUND. Occidental Petroleum Corporation ("Occidental"): - ------------------------------------------------ state of organization: Delaware principal business: engaged, through its subsidiaries and affiliates, in the production, development and marketing of natural resources and chemicals principal business address: 10889 Wilshire Boulevard Los Angeles, California 90024 Occidental C.O.B. Partners ("OXY Partners"): - -------------------------------------------- state of organization: Delaware principal business: general partnership, formed to engage in buying, selling, delivering and dealing in crude oil, and in any and all lawful activities directly or indirectly related or incidental to that business principal business address: 110 West 7th Street P.O. Box 300 Tulsa, Oklahoma 74102 Placid Oil Company ("Placid"): - ------------------------------ state of organization: Delaware principal business: oil and gas exploration and production in the United States Gulf Coast, Canada and the Netherlands principal business address: 10889 Wilshire Boulevard Los Angeles, California 90024 Page 8 of 18 Pages

OXY USA Inc. ("OXY USA"): - ------------------------- state of organization: Delaware principal business: domestic oil and gas exploration and production principal business address: 5 Greenway Plaza Houston, Texas 77046 Occidental Oil and Gas Holding Corporation ("OOGHC"): - ----------------------------------------------------- state of organization: California principal business: holding company for oil and gas entities principal business address: 10889 Wilshire Boulevard Los Angeles, California 90024 Occidental Petroleum Investment Co. ("OPIC"): - --------------------------------------------- state of organization: California principal business: holding company principal business address: 10889 Wilshire Boulevard Los Angeles, California 90024 The name, business address and current principal occupation or employment of each of the executive officers and directors of Occidental, Placid, OXY USA, OOGHC, and OPIC (collectively, the "Companies") are set forth below. Unless otherwise indicated, the business address of each such person is 10889 Wilshire Boulevard, Los Angeles, California 90024. All such persons listed below are citizens of the United States except Mr. Syriani, who is a citizen of Lebanon; Dr. Segovia, who is a citizen of Colombia; and Mr. Feick, who is a citizen of Canada. Unless otherwise indicated, each occupation set forth opposite an individual's name refers to employment with Occidental. Directors of Occidental are identified by an asterisk (*). Name and Business Address Present Principal Occupation or Employment - ------------------------- ------------------------------------------ Ray R. Irani* Chairman of the Board and Chief Executive Officer of Occidental Dale R. Laurance*(3) President of Occidental; Chairman of the Board, President and Chief Executive Officer of OOGHC Ronald W. Burkle* Managing Partner and majority owner of The 9130 West Sunset Boulevard Yucaipa Companies Los Angeles, California 90069 Page 9 of 18 Pages

Name and Business Address Present Principal Occupation or Employment - ------------------------- ------------------------------------------ John S. Chalsty* Senior Advisor, Credit Suisse First Boston 11 Madison Avenue New York, New York 10010 Edward P. Djerejian* Director, James A. Baker III Institute for Rice University, MS-40 Public Policy 6100 Main Street Houston, Texas 77005-1892 R. Chad Dreier* President and Chief Executive Officer, The 24025 Park Sorrento, Suite 400 Ryland Group Calabasas, California 91302 John E. Feick* President and Chief Executive Officer of 230, 319-2nd Avenue, SW Matrix Solutions, Inc. Calgary, Alberta, Canada T2P OC5 Irvin W. Maloney* Retired Chairman and Chief Executive Officer, Dataproducts Corporation Rodolfo Segovia* Member of Executive Committee of Carrera 9A No. 99-02 OF. 1001 Inversiones Sanford Santafe de Bogota, D.C. Colombia Aziz D. Syriani* President and Chief Operating Officer of 505 Park Avenue - 11th Floor The Olayan Group New York, New York 10022 Rosemary Tomich* Owner of the Hope Cattle Company and the A.S. Tomich Construction Company; Chairman of the Board of Directors and Chief Executive Officer, Livestock Clearing, Inc. Walter L. Weisman* Past Chairman and Chief Executive Officer P. O. Box 18017 of American Medical International, Inc. Beverly Hills, California 90209 Stephen I. Chazen Chief Financial Officer and Executive Vice President - Corporate Development of Occidental; Executive Vice President of Placid; Executive Vice President of OXY USA; Executive Vice President of OOGHC Donald P. de Brier Executive Vice President, Secretary and General Counsel of Occidental Page 10 of 18 Pages

Name and Business Address Present Principal Occupation or Employment - ------------------------- ------------------------------------------ Richard W. Hallock Executive Vice President - Human Resources of Occidental J. Roger Hirl Executive Vice President of Occidental 5005 LBJ Freeway Dallas, Texas 75244 John W. Morgan (1)(2) Executive Vice President of Occidental; President of Placid; President of OXY USA; Executive Vice President - Worldwide Production of OOGHC S. P. Dominick, Jr. (4) Vice President and Controller of Occidental; President of OPIC J. R. Havert (4) Vice President and Treasurer of Occidental; Vice President and Treasurer of Placid; Vice President and Treasurer of OXY USA; Vice President and Treasurer of OOGHC; Vice President and Treasurer of OPIC Kenneth J. Huffman Vice President - Investor Relations of Occidental Anthony R. Leach Vice President - Finance of Occidental Robert M. McGee Vice President of Occidental Lawrence P. Meriage Vice President - Communications and Public Affairs of Occidental; Executive Vice President - Communications and Public Affairs of OOGHC Donald L. Moore Vice President and Chief Information 5 East Greenway Plaza Officer of Occidental Houston, Texas 77046-0504 R. Casey Olson Vice President of Occidental; Executive 5 East Greenway Plaza Vice President of OXY USA; Executive Vice Houston, Texas 77046-0504 President of OOGHC Richard A. Swan Vice President - Health, Environment and Safety of Occidental Aurmond A. Watkins, Jr. Vice President - Tax of Occidental; Vice President of OXY USA Page 11 of 18 Pages

Name and Business Address Present Principal Occupation or Employment - ------------------------- ------------------------------------------ Jim A. Leonard (1)(2)(3) Senior Assistant Controller of Occidental; Senior Vice President - Finance of Placid; Senior Vice President - Finance of OXY USA; Senior Vice President - Finance of OOGHC Michael L. Preston (1)(2)(3) Vice President and General Counsel of OOGHC; Vice President and General Counsel of Placid; Vice President and General Counsel of OXY USA John M. Winterman Executive Vice President - Worldwide 5 East Greenway Plaza Exploration and Business Development of Houston, Texas 77046-0504 OOGHC; Executive Vice President - Exploration of Placid; Executive Vice President - Worldwide Exploration of OXY USA James S. Lyerly Executive Vice President - Business Development and Financial Planning & Analysis of OOGHC; Executive Vice President - Business Development and Legal of Placid; Senior Vice President - Business Development and Financial Planning & Analysis of OXY USA Jo Ellen Drisko Vice President - Asset Management of OXY 2480 Fortune Drive, Suite 300 USA; Vice President - Asset Management of Lexington, Kentucky 40509 Placid Ben Langston Vice President - Business Development of 5 East Greenway Plaza OOGHC; Vice President - Business Houston, Texas 77046-0504 Development of Placid; Vice President - Business Development of OXY USA Patrick Oenbring Vice President, Permian Basin of OXY USA; 580 Westlake Park Boulevard Vice President - Permian Basin of Placid Houston, Texas 77079 Paul A. Parsons Vice President - Finance of Placid 580 Westlake Park Boulevard Houston, Texas 77079 Linda S. Peterson (4) Assistant Secretary of Occidental; Vice President and Secretary of Placid; Vice President and Secretary of OXY USA; Vice President and Secretary of OOGHC; Vice President and Secretary of OPIC Stephen J. Pettit Vice President, Legal and Assistant 580 Westlake Park Boulevard Secretary of OXY USA; Vice President, Houston, Texas 77079 Legal and Assistant Secretary of Placid Page 12 of 18 Pages

Name and Business Address Present Principal Occupation or Employment - ------------------------- ------------------------------------------ Damir Vrcek Vice President of OXY USA; Vice President 5 East Greenway Plaza of Placid Houston, Texas 77046-0504 - ----------- (1) Director of Placid (2) Director of OXY USA (3) Director of OOGHC (4) Director of OPIC During the last five years none of the Companies or the natural persons named above (a) has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) (b) was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws. ITEM 3. SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION. On January 29, 2003, pursuant to a commitment letter, dated November 25, 2002, between Occidental C.O.B. Partners, a Delaware general partnership ("Oxy Partners") and Premcor Inc., Oxy Partners and Occidental Petroleum Corporation ("Occidental") purchased 1,025,427 shares and 274,573 shares, respectively, of Premcor Common Stock for $19.20 per share, for an aggregate purchase price of $19,688,198.40 and $5,271,801.60, respectively. The funds for the purchase by Oxy Partners came from an inter-company loan from Occidental. The funds for the purchase by Occidental and the loan to Oxy Partners came from working capital. On June 6, 2002, pursuant to a Warrant Exercise and Share Exchange Agreement, dated as of April 26, 2002 (the "Exercise and Exchange Agreement"), by and among Occidental, Oxy Partners, Blackstone Capital Partners III Merchant Banking Fund L.P., a Delaware limited partnership, Blackstone Offshore Capital Partners III L.P., a Delaware limited partnership, Blackstone Family Investment Partnership III L.P., a Delaware limited partnership, Premcor and Sabine River Holding Corp., a Delaware corporation ("Sabine") and a majority-owned subsidiary of Premcor, Occidental exchanged 681,818 shares of the Common Stock of Sabine for 1,363,636 shares of Premcor Common Stock. The remaining 6,371,010 shares of Premor Common Stock held by Occidental and its affiliate, Oxy Partners, were acquired from Premcor in a series of transactions beginning in 1995 prior to Premcor becoming a publicly traded company. Page 13 of 18 Pages

ITEM 4. PURPOSE OF THE TRANSACTION. Directly and through Oxy Partners, Occidental currently holds its Premcor Shares for investment purposes only with a view toward maximizing long-term shareholder value for Occidental's shareholders and not for the purpose of controlling Premcor. The purchase of additional shares in January 2003 was made to provide a portion of the funds necessary for Premcor to acquire certain refining assets from The Williams Companies. In addition, the purchase maintained Occidental's and Oxy Partner's ownership percentages in Premcor at a level sufficient to avoid a deemed change in Premcor's ownership that would limit Premcor's use of its net operating losses. Other than as set forth in Item 6 of this Amendment 1 to Schedule 13D, Occidental currently has no plans which relate to or would result in or any of the actions listed in clauses (a) through (j) of Item 4 of Schedule 13D.. However, Occidental reviews and analyzes on a continuing basis its investments in each of its subsidiaries and other operations, including the Premcor Shares, in order to determine whether shareholder value for Occidental's shareholders is better served by holding those investments, increasing or disposing of or monetizing those investments or recapitalizing or otherwise restructuring those investments. With respect to the Premcor Shares, these reviews and analyses are based upon a variety of factors, including without limitation, the price of, and other market conditions relating to, the Premcor Shares, the investment return on the Premcor Shares, Premcor's business and prospects, other investment and business opportunities available to Occidental, general stock market and economic conditions, tax considerations and other factors deemed relevant. ITEM 5. INTEREST IN SECURITIES OF THE ISSUER. (a) Occidental: aggregate number of shares held: 9,034,646 ----------- percentage of class: 12.31% Oxy Partners: aggregate number of shares held: 7,126,437 ------------- percentage of class: 9.71% Placid: aggregate number of shares held: 7,126,437 ------- percentage of class: 9.71% OXY USA: aggregate number of shares held: 7,126,437 -------- percentage of class: 9.71% OOGHC: aggregate number of shares held: 7,126,437 ------ percentage of class: 9.71% OPIC: aggregate number of shares held: 7,126,437 ----- percentage of class: 9.71% Page 14 of 18 Pages

(b) Occidental: sole power to vote or direct the vote: 1,908,209 ----------- shared power to vote or direct the vote: 7,126,437 sole power to dispose or direct the disposition: 1,908,209 shared power to dispose or direct the disposition: 7,126,437 Oxy Partners: sole power to vote or direct the vote: 0 ------------- shared power to vote or direct the vote: 7,126,437 sole power to dispose or direct the disposition: 0 shared power to dispose or direct the disposition: 7,126,437 Placid: sole power to vote or direct the vote: 0 ------- shared power to vote or direct the vote: 7,126,437 sole power to dispose or direct the disposition: 0 shared power to dispose or direct the disposition: 7,126,437 OXY USA: sole power to vote or direct the vote: 0 -------- shared power to vote or direct the vote: 7,126,437 sole power to dispose or direct the disposition: 0 shared power to dispose or direct the disposition: 7,126,437 OOGHC: sole power to vote or direct the vote: 0 ------ shared power to vote or direct the vote: 7,126,437 sole power to dispose or direct the disposition: 0 shared power to dispose or direct the disposition: 7,126,437 OPIC: sole power to vote or direct the vote: 0 ----- shared power to vote or direct the vote: 7,126,437 sole power to dispose or direct the disposition: 0 shared power to dispose or direct the disposition: 7,126,437 (c) On January 29, 2003, pursuant to a commitment letter, dated November 25, 2002, between Occidental C.O.B. Partners, a Delaware general partnership ("Oxy Partners") and Premcor Inc., Oxy Partners and Occidental Petroleum Corporation ("Occidental") purchased, in connection with Premcor's public offering of 12.5 million shares, 1,025,427 shares and 274,573 shares, respectively, of Premcor Common Stock for $19.20 per share. ITEM 6. CONTRACTS, ARRANGEMENTS, UNDERSTANDINGS OR RELATIONSHIPS WITH RESPECT TO SECURITIES OF THE ISSUER. Pursuant to the Second Amended and Restated Stockholders' Agreement, dated as of November 3, 1997 (the "Amended Stockholders' Agreement"), between Oxy Partners and Premcor, which by the terms of Exercise and Exchange Agreement became applicable to the Premcor Shares acquired by Occidental upon Exchange of its Sabine Shares, the Premcor Shares held by Occidental and its affiliates are not transferable except in compliance with the Amended Stockholders' Agreement. In addition, the Amended Stockholders' Agreement (i) provides that, so long as Occidental and its affiliates own at least 10% of the fully diluted shares of Premcor, Occidental has the right to designate one director for election to the board, (ii) gives Premcor a Page 15 of 18 Pages

right of first refusal to acquire the shares in the event Occidental receives an offer from a third party to purchase the shares for cash or cash equivalents, and (iii) grants registration rights to Occidental if Premcor is a public company. Except for the Stockholders' Agreement, there are currently no other effective contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 and between such person and any other person with respect to any securities of Premcor. ITEM 7. MATERIAL TO BE FILED AS EXHIBITS. Exhibit 1 Agreement Pursuant to Rule 13d-1(k) Exhibit 2 Second Amended and Restated Stockholders' Agreement, dated as of November 3, 1997, between Oxy Partners and Premcor, Inc.* Exhibit 3 Form of Warrant Exercise and Share Exchange Agreement, dated as of April 26, 2002 (the "Exercise and Exchange Agreement"), by and among Occidental Oxy Partners, Blackstone Capital Partners III Merchant Banking Fund L.P., a Delaware limited partnership, Blackstone Offshore Capital Partners III L.P., a Delaware limited partnership, Blackstone Family Investment Partnership III L.P., a Delaware limited partnership, Premcor and Sabine* Exhibit 4 Commitment Letter, dated November 25, 2002 - ---------- *Previously filed. Page 16 of 18 Pages

SIGNATURE After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. February 5, 2003 OCCIDENTAL PETROLEUM CORPORATION By: /s/ STEPHEN I. CHAZEN ----------------------------------------------- Name: Stephen I. Chazen Title: Chief Financial Officer and Executive Vice President - Corporate Development OCCIDENTAL C.O.B. PARTNERS by OXY USA Inc., its managing partner By: /s/ STEPHEN I. CHAZEN ----------------------------------------------- Name: Stephen I. Chazen Title: Executive Vice President PLACID OIL COMPANY By: /s/ STEPHEN I. CHAZEN ----------------------------------------------- Name: Stephen I. Chazen Title: Executive Vice President OXY USA INC. By: /s/ STEPHEN I. CHAZEN ----------------------------------------------- Name: Stephen I. Chazen Title: Executive Vice President OCCIDENTAL OIL AND GAS HOLDING CORPORATION By: /s/ STEPHEN I. CHAZEN ----------------------------------------------- Name: Stephen I. Chazen Title: Executive Vice President Page 17 of 18 Pages

OCCIDENTAL PETROLEUM INVESTMENT CO. By: /s/ J. R. HAVERT ----------------------------------------------- Name: J. R. Havert Title: Vice President and Treasurer Page 18 of 18 Pages

                                                                       EXHIBIT 1

                                    AGREEMENT

     Pursuant to Rule 13d-1(k) promulgated under the Securities Exchange Act of
1934, as amended, the undersigned agree that the Amendment No. 1 to Schedule 13D
to which this Agreement is attached as Exhibit 1 is filed on behalf of each of
us. This Agreement may be executed in two or more counterparts, each of which
shall be deemed an original, but all of which together shall constitute one and
the same agreement.


February 5, 2003              OCCIDENTAL PETROLEUM CORPORATION


                              By: /s/ STEPHEN I. CHAZEN
                                 -----------------------------------------------
                                 Name:  Stephen I. Chazen
                                 Title: Chief Financial Officer and Executive
                                        Vice President - Corporate Development


                              OCCIDENTAL C.O.B. PARTNERS
                              by OXY USA Inc., its managing partner


                              By: /s/ STEPHEN I. CHAZEN
                                 -----------------------------------------------
                                 Name:  Stephen I. Chazen
                                 Title: Executive Vice President


                              PLACID OIL COMPANY


                              By: /s/ STEPHEN I. CHAZEN
                                 -----------------------------------------------
                                 Name:  Stephen I. Chazen
                                 Title: Executive Vice President


                              OXY USA INC.


                              By: /s/ STEPHEN I. CHAZEN
                                 -----------------------------------------------
                                 Name:  Stephen I. Chazen
                                 Title: Executive Vice President


                                Page 1 of 2 Pages

OCCIDENTAL OIL AND GAS HOLDING CORPORATION By: /s/ STEPHEN I. CHAZEN ----------------------------------------------- Name: Stephen I. Chazen Title: Executive Vice President OCCIDENTAL PETROLEUM INVESTMENT CO. By: /s/ J. R. HAVERT ----------------------------------------------- Name: J. R. Havert Title: Vice President and Treasurer Page 2 of 2 Pages

                                                                       EXHIBIT 4


                           OCCIDENTAL C.O.B. PARTNERS
                            10889 WILSHIRE BOULEVARD
                          LOS ANGELES, CALIFORNIA 90024



November 25, 2002

Premcor Inc.
1700 East Putnam Avenue
Old Greenwich, Connecticut  06870

Gentlemen:

In connection with your acquisition of certain refining assets from Williams
Refining and Marketing, L.L.C., you have advised us that presently you intend to
raise about $220,000,000 of equity through the offering of your common stock.
Subject to the approval of the Board of Directors of Occidental Petroleum
Corporation, we hereby commit to you that we will purchase $30,000,000 of equity
in this offering, concurrently with the closing of such equity offering at the
lesser of the offering price minus the underwriting discount or such price per
share as we may mutually agree.

                                   Very truly yours,

                                   OCCIDENTAL C.O.B. PARTNERS

                                   By:  OXY USA, INC., managing partner

                                   By: /s/ S.I. CHAZEN
                                      ---------------------------------
                                        S. I. Chazen
                                        Executive Vice President