<PAGE>   1
                                                                    EXHIBIT 4.11


                 THIRD SUPPLEMENTAL INDENTURE TO 1997 INDENTURE


         THIRD SUPPLEMENTAL INDENTURE, dated as of August 31, 1998 (the "Third
Supplemental Indenture"), to the 1997 Indenture (as defined below), among
OUTDOOR SYSTEMS, INC., a Delaware corporation (the "Company"), the Guarantors
(as defined in the 1997 Indenture), the subsidiaries of the Company listed on
Schedule A annexed hereto (the "Additional Guarantors") and THE BANK OF NEW
YORK, a New York banking corporation, as trustee (together with any successor
trustee appointed in accordance with the terms of the 1997 Indenture, the
"Trustee").

                              W I T N E S S E T H:

         WHEREAS, the Company has issued its 8-7/8% Senior Subordinated Notes
due 2007 (the "Securities") in the aggregate principal amount of $500,000,000
under and pursuant to the Indenture, dated as of June 23, 1997, among the
Company, the Guarantors named therein and the Trustee, as amended and
supplemented by the First Supplemental Indenture dated September 30, 1997, among
the Company, the Guarantors named therein, the Additional Guarantors named
therein and the Trustee and the Second Supplemental Indenture dated January 22,
1998, among the Company, the Guarantors named therein, the Additional Guarantor
named therein and the Trustee (the "1997 Indenture"); and

         WHEREAS, the Additional Guarantors have become Restricted Subsidiaries
and pursuant to Section 4.21 of the 1997 Indenture are obligated to enter into
this Third Supplemental Indenture, and thereby become Guarantors (as defined in
the 1997 Indenture) as provided in Article X of the 1997 Indenture; and

         WHEREAS, OS Baseline, Inc., formerly an Arizona corporation, Decade
Communications Group, Inc., formerly a Colorado corporation, and Bench
Advertising Company of Colorado, Inc., formerly a Colorado corporation, each a
Guarantor named in the 1997 Indenture, are no longer Guarantors to the 1997
Indenture as each no longer exists as a legal entity pursuant to a merger of
each with and into Outdoor Systems, Inc., a Delaware corporation, on June 30,
1998;

         WHEREAS, OS Advertising of Texas Painting, Inc., formerly a Texas
corporation, a Guarantor named in the 1997 Indenture, was dissolved on June 24,
1998 and is no longer a Guarantor named in the 1997 Indenture;

         WHEREAS, pursuant to Section 8.01(4) of the 1997 Indenture, the
Company, the Guarantors, the Additional Guarantors and the Trustee may enter
into this Third Supplemental Indenture without the consent of any Holder; and

         WHEREAS, all consents and notices required to be obtained and given as
conditions to the execution of this Third Supplemental Indenture pursuant to the
1997 Indenture and all other documents relating to the Securities have been
obtained and given;

         NOW, THEREFORE, for and in consideration of the premises and the mutual
covenants and agreements hereinafter set forth, the parties hereto agree as
follows:
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                                   ARTICLE I.

                           AUTHORIZATION; DEFINITIONS

         Section 1.01. Third Supplemental Indenture. This Third Supplemental
Indenture is supplemental to, and is entered into in accordance with Section
8.01 of, the 1997 Indenture, and except as modified, amended and supplemented by
this Third Supplemental Indenture, the provisions of the 1997 Indenture are in
all respects ratified and confirmed and shall remain in full force and effect.

         Section 1.02. Definitions. Unless the context shall otherwise require,
all terms which are defined in Section 1.01 of the 1997 Indenture shall have the
same meanings, respectively, in this Third Supplemental Indenture as such terms
are given in said Section 1.01 of the 1997 Indenture.

                                   ARTICLE II.

                              ADDITIONAL GUARANTORS

         Section 2.01. Additional Guarantors. Pursuant to Section 10.04 of the
1997 Indenture, the Additional Guarantors (as defined in the Preamble of this
Third Supplemental Indenture) hereby expressly assume the obligations of, and
otherwise agree to perform all of the duties of, a Guarantor under the 1997
Indenture, subject to the terms and conditions thereof, as of the date set forth
opposite the name of such Additional Guarantors on Schedule A hereto.

                                  ARTICLE III.

         Section 3.01. Effective Date. This Third Supplemental Indenture shall
become effective upon execution and delivery hereof.

         Section 3.02. Counterparts. This Third Supplemental Indenture may be
executed in several counterparts, each of which shall be an original and all of
which shall constitute but one and the same instrument.

         Section 3.03. Acceptance. The Trustee accepts the 1997 Indenture, as
supplemented by this Third Supplemental Indenture, and agrees to perform the
same upon the terms and conditions set forth therein as so supplemented. The
Trustee shall not be responsible in any manner whatsoever for or in respect of
the validity or sufficiency of this Third Supplemental Indenture or the due
execution by the Company, the Guarantors or the Additional Guarantors, or for or
in respect of the recitals contained herein, all of which are made by the
Company solely.

         Section 3.04. Successors and Assigns. All covenants and agreements in
this Third Supplemental Indenture by the Company, the Guarantors, the Additional
Guarantors or the Trustee shall bind its respective successors and assigns,
whether so expressed or not.


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         Section 3.05. Severability. In case any provision in this Third
Supplemental Indenture shall be invalid, illegal or unenforceable, the validity,
legality and enforceability of the remaining provisions shall not in any way be
affected or impaired thereby.

         Section 3.06. Governing Law. This Third Supplemental Indenture shall be
governed by and construed in accordance with the internal laws of the State of
New York, without regard to conflicts of laws provisions thereof.

         Section 3.07. Incorporation into 1997 Indenture. All provisions of this
Third Supplemental Indenture shall be deemed to be incorporated in, and made a
part of, the 1997 Indenture; and the 1997 Indenture, as amended and supplemented
by this Third Supplemental Indenture, shall be read, taken and construed as one
and the same instrument.



         IN WITNESS WHEREOF, the parties have caused this Third Supplemental
Indenture to be duly executed, all as of the date first above written.


                                        OUTDOOR SYSTEMS, INC.


                                        By:  /s/ William S. Levine
                                             ----------------------------
                                             Name:  William S. Levine
                                             Title: Chairman of the Board
ATTEST:

/s/  Bill M. Beverage
Bill M. Beverage
Secretary

                    [SIGNATURES CONTINUED ON FOLLOWING PAGE]


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                                   GUARANTORS:

                                   OUTDOOR SYSTEMS PAINTING, INC.

                                   NEW YORK SUBWAYS ADVERTISING CO.,
                                    INC.

                                   OS BUS, INC.

                                   OUTDOOR SYSTEMS (NEW YORK), INC.

                                   NATIONAL ADVERTISING COMPANY

                                   PACIFIC CONNECTION, INC.

                                   SALM ENTERPRISES, INC.

                                   ATLANTA BUS SHELTERS
                                       BY: OUTDOOR SYSTEMS, INC.,
                                       GENERAL PARTNER



                                   By: /s/ William S. Levine
                                       ----------------------------
                                       Name: William S. Levine
                                       Title: Chairman of the Board

ATTEST:

/s/ Bill M. Beverage
--------------------
Bill M. Beverage
Secretary



                    [SIGNATURES CONTINUED ON FOLLOWING PAGE]


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                                   ADDITIONAL GUARANTORS:

                                   ATLANTIC PROSPECT, INC.

                                   OS FLORIDA, INC.

                                   PREMIER SPORTS MARKETING, INC.


                                   By: /s/ William S. Levine
                                       ----------------------------
                                       Name: William S. Levine
                                       Title: Chairman of the Board


ATTEST:

/s/ Bill M. Beverage
--------------------
Bill M. Beverage
Secretary

                                   THE BANK OF NEW YORK, as Trustee


                                   By: /s/ Sandra Carreker
                                       ---------------------
                                       Name: Sandra Carreker
                                       Title: Agent


ATTEST:

/s/ Heidi Van Horn-Bash
-----------------------
Name:  Heidi Van Horn-Bash
Title: Agent


                                      -5-
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                                   SCHEDULE A

                              ADDITIONAL GUARANTORS


Name                                                               Date

Atlantic Prospect, Inc., a Delaware corporation              August 31, 1998

OS Florida, Inc., a Florida corporation                      August 31, 1998

Premier Sports Marketing, Inc., a Delaware corporation       August 31, 1998


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