<PAGE>
 
    
 AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON OCTOBER 8, 1997.     
 
                                                     REGISTRATION NO. 333-31279
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                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549
 
                               ---------------
                                
                             AMENDMENT NO. 3     
                                      TO
                                   FORM S-3
 
            REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933
 
                               ---------------
 
                             MCMORAN OIL & GAS CO.
            (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)
 
         DELAWARE              1615 POYDRAS STREET              72-1266477
      (STATE OR OTHER         NEW ORLEANS, LA 70170          (I.R.S. EMPLOYER
       JURISDICTION              (504) 582-4000             IDENTIFICATION NO.)
    OF INCORPORATION OR 
       ORGANIZATION)      (ADDRESS, INCLUDING ZIP CODE, AND
                                  TELEPHONE NUMBER,
                        INCLUDING AREA CODE, OF REGISTRANT'S
                            PRINCIPAL EXECUTIVE OFFICES)
 
                              JOHN G. AMATO, ESQ.
                                GENERAL COUNSEL
                             MCMORAN OIL & GAS CO.
                              1615 POYDRAS STREET
                         NEW ORLEANS, LOUISIANA 70112
                                (504) 582-4000
(NAME, ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA CODE,
                             OF AGENT FOR SERVICE)
                                  COPIES TO:
                           WILLIAM B. MASTERS, ESQ.
                      JONES, WALKER, WAECHTER, POITEVENT,
                           CARRERE & DENEGRE, L.L.P.
                            201 ST. CHARLES AVENUE
                         NEW ORLEANS, LOUISIANA 70170
 
                               ---------------
 
  APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: As soon as
practicable after this Registration Statement becomes effective.
 
  If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, check the following box.
[_]
 
  If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, other than securities offered only in connection with dividend or
interest reinvestment plans, check the following box. [_]
 
  If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following
box and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. [_]
 
  If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [_]
 
  If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [_]
 
                               ---------------
  THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT
SHALL FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS
REGISTRATION STATEMENT WILL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH
SECTION 8(A) OF THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT
SHALL BECOME EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID
SECTION 8(A), MAY DETERMINE.
 
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<PAGE>
 
                                    PART II
 
                    INFORMATION NOT REQUIRED IN PROSPECTUS
 
ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.
 
  The estimated expenses to be paid by the Registrant in connection with the
securities being registered are as follows:
 
<TABLE>
      <S>                                                             <C>
      SEC registration fee........................................... $   30,346
      Financial Advisory fee.........................................    500,000
      Nasdaq listing fees............................................     18,500
      Legal fees and expenses........................................    265,000
      Subscription/Information Agent fees and expenses...............     35,000
      Accounting fees and expenses...................................     20,000
      Printing expenses..............................................    120,000
      Miscellaneous..................................................     11,154
                                                                      ----------
        Total........................................................ $1,000,000
                                                                      ==========
</TABLE>
 
ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS.
 
  INDEMNIFICATION. The Company is incorporated under the laws of the State of
Delaware. Section 145 of the General Corporation Law of Delaware empowers the
Company to indemnify, subject to the standards therein prescribed, any person
in connection with any action, suit or proceeding brought or threatened by
reason of the fact that such person is or was a director, officer, employee or
agent of the Company. Article Eighth of the Company's Amended and Restated
Certificate of Incorporation (the "Certificate of Incorporation") and Article
XXIV of the Company's By-Laws (the "By-Laws") provide that each person who was
or is made a party to (or is threatened to be made a party to) or is otherwise
involved in any action, suit or proceeding by reason of the fact that such
person is or was a director, officer, employee or agent of the Company shall
be indemnified and held harmless by the Company to the fullest extent
authorized by the General Corporation Law of Delaware against all expenses,
liability and loss (including, without limitation, attorneys' fees, judgments,
fines and amounts paid in settlement) reasonably incurred by such person in
connection therewith. The rights conferred by Article Eighth of the
Certificate of Incorporation and Article XXIV of the By-Laws, as the case may
be, are contractual rights and include the right to be paid by the Company the
expenses incurred in defending such action, suit or proceeding in advance of
the final disposition thereof.
 
  DIRECTOR LIABILITY. Article Eighth of the Certificate of Incorporation
provides that the Company's directors will not be personally liable to the
Company or its stockholders for monetary damages resulting from breaches of
their fiduciary duty as directors except (a) for any breach of the duty of
loyalty to the Company or its stockholders, (b) for acts or omissions not in
good faith or which involve intentional misconduct or a knowing violation of
law, (c) under Section 174 of the General Corporation Law of Delaware, which
makes directors liable for unlawful dividend or unlawful stock repurchases or
redemptions or (d) transactions from which directors derive improper personal
benefit.
 
  INSURANCE. The Company may purchase and maintain insurance to protect itself
and any person eligible to be indemnified hereunder against any liability or
expense asserted or incurred by such person in connection with any action,
whether or not the Company would have the power to indemnify such person
against such liability or expense by law or under the provisions of the
Certificate of Incorporation. The Company has an insurance policy insuring the
Company's directors and officers against certain liabilities, including
liabilities under the Securities Act of 1933.
 
                                     II-1
<PAGE>
 
ITEM 16. EXHIBITS.
 
 5  Opinion of Jones, Walker, Waechter, Poitevent, Carrere & Denegre, L.L.P.,
    as to the legality of the Common Stock.**
 
 8  Opinion of Jones, Walker, Waechter, Poitevent, Carrere & Denegre, L.L.P.,
    as to certain tax matters.**
 
23.1 Consent of Arthur Andersen LLP.*
   
23.2 Consent of Ryder Scott Company.**     
   
23.3 Consent of Crescent Technology, Inc.**     
 
23.4 Consent of Jones, Walker, Waechter, Poitevent, Carrere & Denegre, L.L.P.
   included as part of Exhibits 5 and 8 (refiled for Exhibit 8 opinion).**
 
24  Powers of Attorney.**
 
--------
 *Filed herewith.
**Previously filed.
 
 
                                     II-2
<PAGE>
 
ITEM 17. UNDERTAKINGS
 
  The undersigned Registrant hereby undertakes:
 
  (1) That, for purposes of determining any liability under the Securities
Act, each filing of the Registrant's annual report pursuant to Section 13(a)
or Section 15(d) of the Exchange Act that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such
securities at that time shall be deemed to be the initial bona fide offering
thereof.
 
  (2) Insofar as indemnification for liabilities arising under the Securities
Act may be permitted to directors, officers and controlling persons of the
Registrant pursuant to the foregoing provisions or otherwise, the Registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Securities
Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer or controlling person in
connection with the securities being registered, the Registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act and will be governed by the final adjudication of such issue.
 
                                     II-3
<PAGE>
 
                                  SIGNATURES
   
  Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this Amendment No. 3
to this Registration Statement to be signed on its behalf by the undersigned,
thereunto duly authorized, in New Orleans, Louisiana, on the 8th day of
October, 1997.     
 
                                          McMoRan OIL & GAS CO.
                                          (Registrant)
 
                                                   /s/ Richard C. Adkerson
                                          By: ---------------------------------
                                                     Richard C. Adkerson
                                                  Co-Chairman of the Board
                                                 and Chief Executive Officer
 
  Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement or Amendment thereto has been signed below by the
following persons in the capacities indicated on the 8th day of October,
1997.
 
            SIGNATURE                                  TITLE
 
                *                      Co-Chairman of the Board and Director
---------------------------------
        James R. Moffett
 
     /s/ Richard C. Adkerson         Co-Chairman of the Board, Chief Executive
---------------------------------               Officer and Director
       Richard C. Adkerson                 (Principal Executive Officer)
                                           (Principal Financial Officer)
 
                *                                    Controller
---------------------------------          (Principal Accounting Officer)
     C. Donald Whitmire, Jr.
 
                *                                     Director
---------------------------------
       Robert W. Bruce III
 
                *                                     Director
---------------------------------
          Robert A. Day
 
                *                                     Director
---------------------------------
    William B. Harrison, Jr.
 
                *                                     Director
---------------------------------
        Bobby Lee Lackey
 
                *                                     Director
---------------------------------
      Gabrielle K. McDonald
 
 
                                      S-1
<PAGE>
 
                *                                     Director
---------------------------------
          George Putnam
 
                *                                     Director
---------------------------------
        R. M. Rankin, Jr.
 
                *                                     Director
---------------------------------
        J. Taylor Wharton
 
                             
*By: /s/ Richard C. Adkerson 
  -----------------------------
       Richard C. Adkerson
 
                                      S-2
<PAGE>
 
                                 EXHIBIT INDEX
 
<TABLE>   
<CAPTION>
 EXHIBIT
 NUMBER                         DOCUMENT DESCRIPTION
 -------                        --------------------
 <C>     <S>
   5     Opinion of Jones, Walker, Waechter, Poitevent, Carrere & Denegre,
         L.L.P., as to the legality of the Common Stock.**
   8     Opinion of Jones, Walker, Waechter, Poitevent, Carrere & Denegre,
         L.L.P., as to certain federal income tax matters.**
  23.1   Consent of Arthur Andersen LLP.*
  23.2   Consent of Ryder Scott Company.**
  23.3   Consent of Crescent Technology, Inc.**
  23.4   Consent of Jones, Walker, Waechter, Poitevent, Carrere & Denegre,
         L.L.P. included as part of Exhibits 5 and 8 (refiled for Exhibit 8
         opinion).**
  24     Powers of Attorney.**
</TABLE>    
--------
 *Filed herewith.
**Previously filed.
 
                                      S-3