<PAGE>
                                                                 EXHIBIT 10(p)



                                  THIRD AMENDMENT TO
                     FOURTH AMENDED AND RESTATED CREDIT AGREEMENT

     THIS THIRD AMENDMENT TO FOURTH AMENDED AND RESTATED CREDIT AGREEMENT (this
"AMENDMENT") dated as of the 21st day of June, 1999, by and among PLAINS
RESOURCES INC., a Delaware corporation (the "COMPANY"), ING (U.S.) CAPITAL LLC,
successor in interest to ING (U.S.) CAPITAL CORPORATION, as Agent ("AGENT"), and
the Lenders under the Original Agreement (as defined herein).

                                 W I T N E S S E T H:

     WHEREAS,  the Company, Agent and Lenders entered into that certain Fourth
Amended and Restated Credit Agreement dated as of May 22, 1998, as amended by a
First Amendment to Fourth Amended and Restated Credit Agreement dated November
17, 1998 and a Second Amendment to Fourth Amended and Restated Credit Agreement
dated March 15, 1999 (as amended, the "ORIGINAL AGREEMENT") for the purposes and
consideration therein expressed, pursuant to which Lenders became obligated to
make and made loans to the Company as therein provided; and

     WHEREAS, the Company, Agent and Lenders desire to amend the Original
Agreement for the purposes described herein;

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
and agreements contained herein and in the Original Agreement, in consideration
of the loans which may hereafter be made by Lenders to the Company, and for
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto do hereby agree as follows:

                       ARTICLE I. -- DEFINITIONS AND REFERENCES

     Section 1.1.   TERMS DEFINED IN THE ORIGINAL AGREEMENT.  Unless the context
otherwise requires or unless otherwise expressly defined herein, the terms
defined in the Original Agreement shall have the same meanings whenever used in
this Amendment.

     Section 1.2.   OTHER DEFINED TERMS.  Unless the context otherwise requires,
the following terms when used in this Amendment shall have the meanings assigned
to them in this Section 1.2.

          "AMENDMENT" means this Third Amendment to Fourth Amended and Restated
     Credit Agreement.

          "AMENDMENT DOCUMENTS" means this Amendment.

          "CREDIT AGREEMENT" means the Original Agreement as amended hereby.

                                       1

<PAGE>

                              ARTICLE II. -- AMENDMENTS

     Section 2.1.    DEFINITIONS.  The definition of "Revolving Credit
Termination Date" set forth in Section 1.01 of the Original Agreement is hereby
amended in its entirety to read as follows:

          "REVOLVING CREDIT TERMINATION DATE" shall mean the earlier of (a) July
     1, 2001 and (b) the date on which the Commitment is reduced to zero or
     terminated pursuant to Section 2.03 hereof.

     Section 2.2.    REPAYMENT OF TERM LOANS.  Section 3.01(b) of the Original
Agreement is hereby amended in its entirety to read as follows:

          (b)  The Company will repay the principal of the Term Loans in sixteen
     installments payable on each Quarterly Date beginning October 1, 2001, with
     the final installment being due and payable on or before July 1, 2005.
     Each such installment shall be equal to one-sixteenth of the original
     principal amount of the Term Loans as of the Revolving Credit Termination
     Date.  In any event all unpaid principal and interest shall be due and
     payable in full on the final maturity of July 1, 2005.  As set forth in
     Section 2.07(c), all optional and mandatory prepayments made on the Term
     Loans shall be applied to the scheduled installments in inverse order of
     their maturity.

                     ARTICLE III. -- CONDITIONS OF EFFECTIVENESS

     Section 3.1.   EFFECTIVE DATE.  This Amendment shall become effective as of
the date first above written when and only when (i) Agent shall have received,
at Agent's office, a counterpart of this Amendment executed and delivered by the
Company and each Lender, and (ii) Agent shall have additionally received all of
the following documents, each document (unless otherwise indicated) being dated
the date of receipt thereof by Agent, duly authorized, executed and delivered,
and in form and substance satisfactory to Agent:

          (A)  OFFICER'S CERTIFICATE.  A certificate of a duly authorized
     officer of the Company to the effect that all of the representations and
     warranties set forth in Article IV hereof are true and correct at and as of
     the date thereof.

          (B)  SUPPORTING DOCUMENTS.  Such supporting documents as Agent may
     reasonably request.

                    ARTICLE IV. -- REPRESENTATIONS AND WARRANTIES

     Section 4.1.   REPRESENTATIONS AND WARRANTIES OF THE COMPANY.  In order to
induce Agent and Lenders to enter into this Amendment, the Company represents
and warrants to Agent and Lenders that:

          (a)  The representations and warranties contained in Section 7 of the
     Original Agreement, are true and correct at and as of the time of the
     effectiveness hereof, subject

                                       2

<PAGE>

     to the amendment of certain of the Schedules to the Credit Agreement as
     attached hereto.  No Default has occurred and is continuing.

          (b)  The Company and the Subsidiaries are duly authorized to execute
     and deliver this Amendment and the other Amendment Documents to the extent
     a party thereto, and the Company is and will continue to be duly authorized
     to borrow and perform its obligations under the Credit Agreement.  The
     Company and the Subsidiaries have duly taken all corporate action necessary
     to authorize the execution and delivery of this Amendment and the other
     Amendment Documents, to the extent a party thereto, and to authorize the
     performance of their respective obligations thereunder.

          (c)  The execution and delivery by the Company and the Subsidiaries of
     this Amendment and the other Amendment Documents, to the extent a party
     thereto, the performance by the Company and the Subsidiaries of their
     respective obligations hereunder and thereunder, and the consummation of
     the transactions contemplated hereby and thereby, do not and will not
     conflict with any provision of law, statute, rule or regulation or of the
     certificate or articles of incorporation and bylaws of the Company or any
     Subsidiary, or of any material agreement, judgment, license, order or
     permit applicable to or binding upon the Company or any Subsidiary, or
     result in the creation of any lien, charge or encumbrance upon any assets
     or properties of the Company or any Subsidiary, except in favor of Agent
     for the benefit of Lenders.  Except for those which have been duly
     obtained, no consent, approval, authorization or order of any court or
     governmental authority or third party is required in connection with the
     execution and delivery by the Company or any Subsidiary of this Amendment
     or any other Amendment Document, to the extent a party thereto, or to
     consummate the transactions contemplated hereby and thereby.

          (d)  When this Amendment and the other Amendment Documents have been
     duly executed and delivered, each of the Basic Documents, as amended by
     this Amendment and the other Amendment Documents, will be a legal and
     binding instrument and agreement of the Company and the Subsidiaries, to
     the extent a party thereto, enforceable in accordance with its terms,
     (subject, as to enforcement of remedies, to applicable bankruptcy,
     insolvency and similar laws applicable to creditors' rights generally and
     to general principles of equity).

                             ARTICLE V. -- MISCELLANEOUS

     Section 5.1.   RATIFICATION OF AGREEMENTS.  The Original Agreement, as
hereby amended, is hereby ratified and confirmed in all respects.  The Basic
Documents, as they may be amended or affected by this Amendment and/or the other
Amendment Documents, are hereby ratified and confirmed in all respects.  Any
reference to the Credit Agreement in any Basic Document shall be deemed to refer
to this Amendment also.  The execution, delivery and effectiveness of this
Amendment and the other Amendment Documents shall not, except as expressly
provided herein or therein, operate as a waiver of any right, power or remedy of
Agent or any Lender under the Credit Agreement or any other Basic Document nor
constitute a waiver of any provision of the Credit Agreement or any other Basic
Document.

                                       3

<PAGE>

     Section 5.2.   RATIFICATION OF SECURITY DOCUMENTS.  The Company, Agent and
Lenders each acknowledge and agree that any and all indebtedness, liabilities or
obligations arising under or in connection with the Notes are Obligations and is
secured indebtedness under, and is secured by, each and every Security Document
to which the Company is a party.  The Company hereby re-pledges, re-grants and
re-assigns a security interest in and lien on every asset of the Company
described as collateral in any Security Document.

     Section 5.3.   SURVIVAL OF AGREEMENTS.  All representations, warranties,
covenants and agreements of the Company herein and in the other Amendment
Documents shall survive the execution and delivery of this Amendment and the
other Amendment Documents and the performance hereof and thereof, including
without limitation the making or granting of each Loan, and shall further
survive until all of the Obligations are paid in full.  All statements and
agreements contained in any certificate or instrument delivered by the Company
or any Subsidiary hereunder, under the other Amendment Documents or under the
Credit Agreement to Agent or any Lender shall be deemed to constitute
representations and warranties by, or agreements and covenants of, the Company
under this Amendment and under the Credit Agreement.

     Section 5.4.   BASIC DOCUMENTS.  This Amendment and each of the other
Amendment Documents is a Basic Document, and all provisions in the Credit
Agreement pertaining to Basic Documents apply hereto and thereto.

     Section 5.5.   GOVERNING LAW.  THIS AMENDMENT AND THE OTHER AMENDMENT
DOCUMENTS SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE
STATE OF NEW YORK AND ANY APPLICABLE LAWS OF THE UNITED STATES OF AMERICA IN ALL
RESPECTS, INCLUDING CONSTRUCTION, VALIDITY AND PERFORMANCE.

     Section 5.6.   COUNTERPARTS.  This Amendment and each of the other
Amendment Documents may be separately executed in counterparts and by the
different parties hereto in separate counterparts, each of which when so
executed shall be deemed to constitute one and the same Amendment or Amendment
Document, as the case may be.

     IN WITNESS WHEREOF, this Amendment is executed as of the date first above
written.

                                   PLAINS RESOURCES INC.

                                   By: /s/ Michael R. Patterson
                                      --------------------------------------
                                        Name: Michael R. Patterson
                                        Title: Vice President

                                       4

<PAGE>

                                   ING (U.S.) CAPITAL LLC,
                                   as Agent and a Lender

                                   By: /s/ Peter Y. Clinton
                                      --------------------------------------
                                        Name: Peter Y. Clinton
                                        Title: Senior Vice President


                                   BANKBOSTON, N.A., Lender

                                   By: /s/ Terrence Ronan
                                      --------------------------------------
                                        Terrence Ronan, Vice President


                                   DEN NORSKE BANK ASA, Lender

                                   By: /s/ Byron L. Cooley
                                      --------------------------------------
                                        Name: Byron L. Cooley
                                        Title: Senior Vice President

                                   By: /s/ J. Morten Kreutz
                                      --------------------------------------
                                        Name: J. Morten Kreutz
                                        Title: First Vice President


                                   WELLS FARGO BANK (TEXAS),
                                   NATIONAL ASSOCIATION, Lender

                                   By: /s/ Ann M. Rhoads
                                      --------------------------------------
                                        Name: Ann M. Rhoads
                                        Title: Vice President


                                   CHASE BANK OF TEXAS, N.A., Lender

                                   By: /s/ Russell A. Johnson
                                      --------------------------------------
                                        Name: Russell A. Johnson
                                        Title: Vice President


                                   COMERICA BANK-TEXAS, Lender

                                   By: /s/ Daniel G. Steele
                                      --------------------------------------
                                        Name: Daniel G. Steele
                                        Title: Senior Vice President

                                   MEESPIERSON CAPITAL CORP., Lender

                                       5

<PAGE>

                                   By: /s/ Darrell W. Holley
                                      --------------------------------------
                                        Name: Darrell W. Holley
                                        Title: Senior Vice President

                                   By: /s/ Richard F. Herrick
                                      --------------------------------------
                                        Name: Richard F. Herrick
                                        Title: Senior Vice President


                                   BANK OF SCOTLAND, Lender

                                   By: /s/ Annie Glynn
                                      --------------------------------------
                                        Name: Annie Glynn
                                        Title: Senior Vice President


                                   U.S. BANK NATIONAL ASSOCIATION, Lender

                                   By: /s/ Charles S. Searle
                                      --------------------------------------
                                        Name: Charles S. Searle
                                        Title: Senior Vice President


                                   HIBERNIA NATIONAL BANK

                                   By: /s/ Spencer Gagnet
                                      --------------------------------------
                                        Name: Spencer Gagnet
                                        Title: Vice President


                                   GENERAL ELECTRIC CAPITAL CORPORATION

                                   By: /s/ Eric A. Schaefer
                                      --------------------------------------
                                        Name: Eric A. Schaefer
                                        Title: Manager - Operations

                                       6

<PAGE>

                                CONSENT AND AGREEMENT

     Each of the undersigned Subsidiary Guarantors hereby consents to the
provisions of this Amendment and the transactions contemplated herein and hereby
(i) acknowledges and agrees that any and all indebtedness, liabilities or
obligations arising under or in connection with the Notes are Obligations and
are secured indebtedness under, and are secured by, each and every Security
Document to which it is a party, (ii) re-pledges, re-grants and re-assigns a
security interest in and lien on all of its assets described as collateral in
any Security Document, (iii) ratifies and confirms its Amended and Restated
Guaranty dated May 22, 1998 made by it for the benefit of Agent and Lenders, and
(iv) expressly acknowledges and agrees that such Subsidiary Guarantor guarantees
all indebtedness, liabilities and obligations arising under or in connection
with the Notes pursuant to the terms of such Amended and Restated Guaranty, and
agrees that its obligations and covenants thereunder are unimpaired hereby and
shall remain in full force and effect.

                                  PLAINS RESOURCES INTERNATIONAL INC.
                                  STOCKER RESOURCES, INC.
                                  CALUMET FLORIDA, INC.
                                  PLAINS ILLINOIS INC.


                                   By: /s/ Michael R. Patterson
                                      --------------------------------------
                                        Name: Michael R. Patterson
                                        Title: Vice President

                                   STOCKER RESOURCES, L.P.

                                   By:  Stocker Resources, Inc.,
                                         its General Partner


                                        By: /s/ Michael R. Patterson
                                           ----------------------------------
                                             Name: Michael R. Patterson
                                             Title: Vice President

                                       7