<PAGE>

                                                                  EXHIBIT (4)(a)

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                                  ONEOK, INC.

                         7.125% Senior Notes due 2011

                                _______________

                         EIGHTH SUPPLEMENTAL INDENTURE

                           Dated as of April 6, 2001

                                _______________

                           The Chase Manhattan Bank
                                    TRUSTEE

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<PAGE>

     THIS EIGHTH SUPPLEMENTAL INDENTURE is made as of the 6th day of April,
2001, by and between ONEOK, INC., an Oklahoma corporation (the "Company"), and
THE CHASE MANHATTAN BANK, a New York banking corporation formerly known as Chase
Bank of Texas, National Association (the "Trustee").

                             W I T N E S S E T H:

     WHEREAS, the Company has heretofore entered into an Indenture, dated as of
September 24, 1998 (the "Original Indenture"), with the Trustee;

     WHEREAS, the Original Indenture is incorporated herein by this reference
and the Original Indenture, as supplemented by this Eighth Supplemental
Indenture, is herein called the "Indenture";

     WHEREAS, under the Original Indenture, a new series of Securities may at
any time be established pursuant to a supplemental indenture executed by the
Company and the Trustee;

     WHEREAS, the Company proposes to create under the Indenture a new series of
Securities; and

     WHEREAS, all conditions necessary to authorize the execution and delivery
of this Eighth Supplemental Indenture and to make it a valid and binding
obligation of the Company have been done or performed.

     NOW, THEREFORE, in consideration of the agreements and obligations set
forth herein and for other good and valuable consideration, the sufficiency of
which is hereby acknowledged, the parties hereto hereby agree as follows:

                                   ARTICLE 1

                         7.125% SENIOR NOTES DUE 2011

     SECTION 101. Establishment. There is hereby established a new series of
Securities to be issued under the Indenture, to be designated as the Company's
7.125% Senior Notes due 2011 (the "Notes").

     There are to be authenticated and delivered $400,000,000 principal amount
of Notes to be issued to underwriters at 99.262% of principal amount. The
Company shall have the right to issue additional Notes at any time upon
compliance with the provisions of the Indenture applicable to the issuance of
additional Securities. The Notes shall be issued in definitive fully registered
form.

     The Notes shall be issued initially in the form of one or more Global
Securities, each in substantially the form set out in Exhibit A hereto. The
initial Depositary with respect to the Notes shall be the Depository Trust
Company.
<PAGE>

     The Company will not pay Additional Amounts, as defined in Section 1008 of
the Original Indenture.

     The form of the Trustee's Certificate of Authentication for the Notes shall
be in substantially the form set forth in Exhibit B hereto.

     Each Note shall be dated the date of authentication thereof and shall bear
interest from the Original Issue Date.

     The interest rate on the Notes will not be reset pursuant to Section 308(b)
of the Original Indenture and the Stated Maturity shall not be extended pursuant
to Section 309 of the Original Indenture.

     SECTION 102. Definitions. The following defined terms used herein shall,
unless the context otherwise requires, have the meanings specified below.
Capitalized terms used herein for which no definition is provided herein shall
have the meanings set forth in the Original Indenture.

     "Adjusted Treasury Rate" means, with respect to any Redemption Date, the
rate per annum equal to the semi-annual equivalent yield to maturity of the
Comparable Treasury Issue, assuming a price of the Comparable Treasury Issue
(expressed as a percentage of its principal amount) equal to the Comparable
Treasury Price for such Redemption Date.

     "Comparable Treasury Issue" means the United States Treasury security
selected by the Quotation Agent as having a maturity comparable to the remaining
term of the Notes to be redeemed that would be utilized, at the time of
selection and in accordance with customary financial practice, in pricing new
issues of corporate debt securities of comparable maturity to the remaining term
of the Notes.

     "Comparable Treasury Price" means, with respect to any Redemption Date, the
Reference Treasury Dealer Quotation for such Redemption Date.

     "Interest Payment Dates" means April 15 and October 15 of each year,
commencing October 15, 2001.

     "Original Issue Date" means April 6, 2001.

     "Quotation Agent" means the Reference Treasury Dealer.

     "Reference Treasury Dealer" means Banc of America Securities LLC and its
successors; provided, however, that if the foregoing shall cease to be a primary
U.S. Government Securities dealer in New York City (a "Primary Treasury
Dealer"), the Company shall substitute therefor another Primary Treasury Dealer
and certify same to the Trustee.

     "Reference Treasury Dealer Quotations" means, with respect to any
Redemption Date, the average, as determined by the Company and certified to the
Trustee by the Company, of the bid

                                       2
<PAGE>

and asked prices for the Comparable Treasury Issue (expressed in each case as a
percentage of its principal amount) quoted in writing to the Company by the
Reference Treasury Dealer at 5:00 p.m. on the third Business Day preceding such
Redemption Date.

     "Regular Record Date" means April 1 in the case of an April 15 Interest
Payment Date and October 1 in the case of an October 15 Interest Payment Date.

     "Stated Maturity" means April 15, 2011.

     SECTION 103. Payment of Principal and Interest. The principal of the Notes
shall be due at Stated Maturity (unless earlier redeemed). The unpaid principal
amount of the Notes shall bear interest at the rate of 7.125% per annum until
paid or duly provided for. Interest shall be paid semi-annually in arrears on
each Interest Payment Date to the Person in whose name the Notes are registered
on the Regular Record Date for such Interest Payment Date and on Stated
Maturity.  Accrued interest paid on Stated Maturity shall be paid to the Person
to whom principal is paid.  Any such interest that is not so punctually paid or
duly provided for will forthwith cease to be payable to the Holders on such
Regular Record Date and may be paid to the Person or Persons in whose name the
Notes are registered at the close of business on a Special Record Date for the
payment of such defaulted interest to be fixed by the Trustee, notice whereof
shall be given to Holders of the Notes not less than ten days prior to such
Special Record Date.

     Payments of interest on the Notes will include interest accrued to but
excluding the respective Interest Payment Dates. Interest payments for the Notes
shall be computed and paid on the basis of a 360-day year of twelve 30-day
months.

     Payment of the principal and interest due at the Stated Maturity or earlier
redemption of the Notes shall be made upon surrender of the Notes at the
Corporate Trust Office of the Trustee. The principal of and interest on the
Notes shall be paid in such currency of the United States of America as at the
time of payment is legal tender for payment of public and private debts.
Payments of interest will be made at the option of the Company, by (i) check
mailed to the address of the Person entitled thereto as such address shall
appear in the Security Register or (ii)  wire transfer to an account located in
the United States maintained by the payee.

     SECTION 104. Denominations. The Notes may be issued in denominations of
$1,000 or any integral multiple thereof.

     SECTION 105. Global Securities. The Notes will be issued in the form of one
or more Global Securities registered in the name of the Depositary or its
nominee. Except under the limited circumstances described below, Notes
represented by a Global Security will not be exchangeable for, and will not
otherwise be issuable as, Notes in definitive form. The Global Securities may
not be transferred except by the Depositary to a nominee of the Depositary or by
a nominee of the Depositary to the Depositary or another nominee of the
Depositary or to a successor Depositary or its nominee.

     Owners of beneficial interests in a Global Security will not be considered
the Holders thereof for any purpose under the Indenture, and no Global Security
representing a Note shall be

                                       3
<PAGE>

exchangeable, except for another Global Security of like denomination and tenor
to be registered in the name of the Depositary or its nominee or to a successor
Depositary or its nominee. The rights of Holders of such Global Security shall
be exercised only through the Depositary.

     A Global Security shall be exchangeable for Notes registered in the names
of persons other than the Depositary or its nominee only if (i) the Depositary
notifies the Company that it is unwilling or unable to continue as a Depositary
for such Global Security and no successor Depositary shall have been appointed
by the Company, or if at any time the Depositary ceases to be a clearing agency
registered under the Securities Exchange Act of 1934, as amended, at a time when
the Depositary is required to be so registered to act as such Depositary and no
successor Depositary shall have been appointed by the Company, in each case
within 90 days after the Company receives such notice or becomes aware of such
cessation, (ii) the Company in its sole discretion determines that such Global
Security shall be so exchangeable, or (iii) there shall have occurred an Event
of Default with respect to the Notes.

     SECTION 106. Transfer. No service charge will be made for any transfer or
exchange of Notes, but payment will be required of a sum sufficient to cover any
tax or other governmental charge that may be imposed in connection therewith.

     SECTION 107. Redemption at the Company's Option. The Notes will be
redeemable, in whole or in part, at any time at the option of the Company at a
redemption price (the "Redemption Price") equal to the greater of (i) 100% of
the principal amount of such Notes or (ii) as determined by a Quotation Agent,
the sum of the present values of the remaining scheduled payments of principal
and interest (not including any portion of those payments of interest accrued as
of the date of redemption) discounted to the date of redemption (the "Redemption
Date") on a semi-annual basis (assuming a 360-day year consisting of twelve 30-
day months) at the Adjusted Treasury Rate plus 30 basis points, plus, in the
case of (i) and (ii), accrued but unpaid interest to the Redemption Date.

     Notice of any redemption will be mailed at least 30 days but not more than
60 days before the Redemption Date to each Holder of the Notes to be redeemed.
Unless the Company defaults in payment of the Redemption Price, interest will
cease to accrue on the Notes or portions thereof called for redemption on and
after the Redemption Date.

     SECTION 108.  Other Terms.

     The Notes will not have a sinking fund.

     Notice of redemption shall be given as provided in Section 1104 of the
Original Indenture.

     Any redemption of less than all of the Notes shall, with respect to the
principal thereof, be divisible by $1,000.

                                       4
<PAGE>

                                   ARTICLE 2

                           MISCELLANEOUS PROVISIONS

     SECTION 201. Recitals by Company. The recitals in this Eighth Supplemental
Indenture are made by the Company only and not by the Trustee, and all of the
provisions contained in the Original Indenture in respect of the rights,
privileges, immunities, powers and duties of the Trustee shall be applicable in
respect of Notes and of this Eighth Supplemental Indenture as fully and with
like effect as if set forth herein in full.

     SECTION 202. Ratification and Incorporation of Original Indenture. The
Original Indenture is in all respects ratified and confirmed, and the Original
Indenture and this Eighth Supplemental Indenture shall be read, taken and
construed as one and the same instrument; provided that, in the case of a
conflict between this Eighth Supplemental Indenture and the Original Indenture,
this Eighth Supplemental Indenture shall control.

     SECTION 203. Executed in Counterparts. This Eighth Supplemental Indenture
may be simultaneously executed in several counterparts, each of which shall be
deemed to be an original, and such counterparts shall together constitute one
and the same instrument.

     SECTION 204. Parties Interested Herein. Nothing in the Indenture expressed
or implied is intended or shall be construed to confer upon, or to give or grant
to, any person or entity, other than the Company, the Trustee, the Paying Agent
and the registered owners of the Notes, any right, remedy or claim under or by
reason of the Indenture or any covenant, condition or stipulation hereof, and
all covenants, stipulations, promises and agreements in the Indenture contained
by and on behalf of the Company shall be for the sole and exclusive benefit of
the Company, the Trustee, the Paying Agent and the registered owners of the
Notes.

                 [remainder of page intentionally left blank]

                                       5
<PAGE>

          IN WITNESS WHEREOF, each party hereto has caused this Eighth
Supplemental Indenture to be signed in its name and behalf by its duly
authorized officer or signatory, all as of the day and year first above written.

                                    ONEOK, INC.



                                    By:_________________________________________



                                    THE CHASE MANHATTAN BANK, as Trustee


                                    By:_________________________________________
                                             Authorized Signatory

                                       6
<PAGE>

                                                                       EXHIBIT A


                                 FORM OF NOTE


                                  [Attached]

                                      A-1
<PAGE>

     THIS SECURITY IS A GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE
HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF A DEPOSITORY OR A
NOMINEE THEREOF.  THIS SECURITY MAY NOT BE EXCHANGED IN WHOLE OR IN PART FOR A
SECURITY REGISTERED, AND NO TRANSFER OF THIS SECURITY IN WHOLE OR IN PART MAY BE
REGISTERED, IN THE NAME OF ANY PERSON OTHER THAN SUCH DEPOSITORY OR ITS NOMINEE
EXCEPT IN THE LIMITED CIRCUMSTANCES DESCRIBED IN THE INDENTURE.

     UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE
DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION ("DTC"), TO THE ISSUER OR ITS
AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE, OR PAYMENT, AND ANY CERTIFICATE
ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME AS IS
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE TO
CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE, OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.


================================================================================
No. ___


                                  ONEOK, INC.
                         7.125% SENIOR NOTES DUE 2011

                              CUSIP:   682680AL7

     ONEOK, Inc., an Oklahoma corporation (herein called "Company," which term
includes any successor corporation under the Indenture referred to herein), for
value received, hereby promises to pay to:

     CEDE & CO.

or registered assigns, the principal sum of

     *_____________ DOLLARS*

on April 15, 2011 and to pay interest on such principal sum at the rate of seven
and one-eighth percent (7.125%) per annum.

     The Company will pay interest from April 6, 2001, semi-annually on April 15
and October 15 of each year, and on the date of maturity, commencing October 15,
2001 (each such date an "Interest Payment Date"), until the principal hereof is
otherwise paid or duly provided for.  The interest so payable, and punctually
paid or duly provided for, on any Interest Payment Date will, as provided in the
Indenture (as defined below), be paid to the holder of this Note (the
<PAGE>

"Holder") of record at the close of business on the regular record date (the
"Regular Record Date") for such Interest Payment Date, which shall be April 1
(in the case of the April 15 Interest Payment Date) or October 1 (in the case of
the October 15 Interest Payment Date), whether or not a Business Day. Interest
will be computed on the basis of a 360-day year of twelve 30-day months.

     Any such interest not so punctually paid or duly provided for shall
forthwith cease to be payable to the Holder on such Regular Record Date by
virtue of his having been such Holder, and may be paid to the Holder of record
of this Note at the close of business on a special record date (the "Special
Record Date") fixed by the Company for the payment of such defaulted interest,
notice whereof shall be given to Holders not less than ten days prior to such
Special Record Date, all as more fully provided in the Indenture.

     Payment of the principal of this Note and the interest thereof will be made
at the office or agency of the Company in the Borough of Manhattan, City and
State of New York or at the Corporate Trust Office of the Trustee in such
currency of the United States of America as at the time of payment is legal
tender for payment of public and private debts.
<PAGE>

                                  ONEOK, INC.
                         7.125% Senior Notes due 2011

     This Note is one of a duly authorized issue of debt securities of the
Company (herein called the "Securities"), issuable in one or more series, issued
and to be issued under and pursuant to an Indenture dated as of September 24,
1998, as amended and supplemented by that certain Eighth Supplemental Indenture
(the "Indenture"), duly executed and delivered by the Company to The Chase
Manhattan Bank, formerly known as Chase Bank of Texas, National Association, as
trustee (the "Trustee," which term includes any successor trustee under the
Indenture), and is one of a series unlimited in aggregate principal amount and
designated as 7.125% Senior Notes due 2011 (the "Notes"). Reference is hereby
made to the Indenture for a description of the rights, limitations of rights,
obligations, duties and immunities thereunder of the Trustee, the Company and
the holders of Securities (including Holders of the Notes).

     The Notes are subject to defeasance at the option of the Company as
provided in the Indenture.

     As long as this Note is represented in global form (the "Global Security")
registered in the name of the Depository or its nominee, except as provided in
the Indenture and subject to certain limitations therein set forth, no Global
Security shall be exchangeable or transferable.

     If an Event of Default (as defined in the Indenture) with respect to the
Notes shall occur and be continuing, the principal plus any accrued interest may
be declared due and payable in the manner and with the effect and subject to the
conditions provided in the Indenture.

     The Indenture permits the amendment thereof and the modification of the
rights and obligations of the Company and the rights of the Holders under the
Indenture at any time by the Company and the Trustee with the consent of the
Holders of a majority in aggregate principal amount of the Securities then
Outstanding (as defined in the Indenture) of all series that are affected by
such amendment or modification, except that certain amendments that do not
adversely affect the rights of any holder of the Securities may be made without
the approval of holders of the Securities.  No amendment or modification may,
among other things, change the Stated Maturity of any Security, reduce the
principal amount thereof, reduce the rate or change the time of payment of any
interest thereon, or reduce the percentage in principal amount of Securities,
consent of the holders of which is required for any such amendment or
modification, without the consent of each Securityholder affected.

     Notwithstanding any provision in the Indenture or any provision of this
Note, the Holder of this Note shall have the right, which is absolute and
unconditional, to receive payment of the principal of (and premium, if any) and
interest on this Note at the times, places and rate, and in the currency herein
prescribed.

     The Notes may be redeemed, in whole or in part, at any time at the option
of the Company at a Redemption Price equal to the greater of (i) 100% of the
principal amount of such Notes or (ii) as determined by a Quotation Agent, the
sum of the present values of the remaining scheduled payments of principal and
interest (not including any portion of those payments of interest accrued as of
the date of redemption) discounted to the date of redemption on a semi-
<PAGE>

annual basis (assuming a 360-day consisting of twelve 30-day months) at the
Adjusted Treasury Rate plus 30 basis points, plus, in the case of (i) and (ii),
accrued but unpaid interest to the Redemption Date.

     In the event of redemption of this Note in part only, a new Note or Notes
of this series for the unredeemed portion hereof will be issued in the name of
the Holder upon surrender hereof or otherwise reduced in accordance with the
provisions of the Indenture.  The Notes will not have a sinking fund.

     THIS NOTE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS
OF THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF CONFLICTS OF LAWS.

     All terms used in this Note but not defined herein have the meanings
assigned to them in the Indenture.

     Unless the certificate of authentication hereon has been executed by or on
behalf of the Trustee by manual signature, this Note shall not be entitled to
any benefit under the Indenture or be valid or obligatory for any purpose.
<PAGE>

     IN WITNESS WHEREOF, the Company has caused this Note to be duly executed.

                              ONEOK, INC.


                              By:_______________________________________________
                                    Name:
                                    Title:


                              Attested:_________________________________________
                                         Name:
                                         Title:


TRUSTEE'S CERTIFICATE OF AUTHENTICATION

Dated:  ____________________

This is one of the Notes referred to in the within-
mentioned Indenture.

                    THE CHASE MANHATTAN BANK
                      as Trustee


                    By:__________________________________________
                         Authorized Signatory
<PAGE>

                 SCHEDULE OF EXCHANGES OF INTEREST IN THE NOTE

     The following exchanges of interests in this Note have been made:

<TABLE>
<S>                      <C>                      <C>                   <C>                           <C>
                                                                           Principal Amount of
                         Amount of decrease       Amount of increase       this Note following        Signature of authorized
Date of Exchange            in this Note             in this Note       such decrease or increase      signatory of Trustee
</TABLE>
<PAGE>

                                ASSIGNMENT FORM

                 To assign this Note, fill in the form below:
                   I or we assign and transfer this Note to

                               ________________

                               ________________

                  Insert assignee's soc. sec. or tax I.D. no.

                               ________________



________________________________________________________________________________
             (Print or type assignee's name, address and zip code)



________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
and all rights thereunder and irrevocably appoint ______________________________
________________________________________________________________________________
agent to transfer this Note on the books of the Company.  The agent may
substitute another to act for him.

________________________________________________________________________________

Dated: ___________________    __________________________________________________

                              __________________________________________________

     THE SIGNATURE TO THIS ASSIGNMENT MUST CORRESPOND WITH THE NAME AS IT
APPEARS ON THE FIRST PAGE OF THE NOTE.

     THE SIGNATURE MUST BE GUARANTEED BY AN "ELIGIBLE GUARANTOR INSTITUTION"
THAT IS A MEMBER OR PARTICIPANT IN A "SIGNATURE GUARANTEE PROGRAM" (E.G., THE
SECURITIES TRANSFER AGENTS MEDALLION PROGRAM, THE STOCK EXCHANGE MEDALLION
PROGRAM OR THE NEW YORK STOCK EXCHANGE, INC. MEDALLION PROGRAM).
<PAGE>

                                                                       EXHIBIT B


                         CERTIFICATE OF AUTHENTICATION

    This is one of the Notes referred to in the within-mentioned Indenture.

                                    THE CHASE MANHATTAN BANK, as
                                      Trustee [or Successor Trustee]



     Dated:__________________       By:_________________________________________
                                         Authorized Signatory