<PAGE>   1
                                                                     Exhibit 4.2


                           (FORM OF FACE OF SECURITY)

         [IF THIS SECURITY IS A GLOBAL SECURITY INSERT: THIS SECURITY IS A
GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE HEREINAFTER REFERRED TO AND
IS REGISTERED IN THE NAME OF THE DEPOSITORY TRUST COMPANY ("DTC") OR A NOMINEE
OF DTC. THIS SECURITY IS EXCHANGEABLE FOR SECURITIES REGISTERED IN THE NAME OF A
PERSON OTHER THAN DTC OR ITS NOMINEE ONLY IN THE LIMITED CIRCUMSTANCES DESCRIBED
IN THE INDENTURE, AND NO TRANSFER OF THIS SECURITY (OTHER THAN A TRANSFER OF
THIS SECURITY AS A WHOLE BY DTC TO A NOMINEE OF DTC OR BY A NOMINEE OF DTC TO
DTC OR ANOTHER NOMINEE OF DTC) MAY BE REGISTERED EXCEPT IN LIMITED
CIRCUMSTANCES.

         UNLESS THIS SECURITY IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF
DTC TO THE ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR
PAYMENT, AND ANY SECURITY ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN
SUCH OTHER NAME AS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY
PAYMENT HEREON 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 IN AS MUCH AS THE
REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.]

         THE SECURITIES WILL BE ISSUED AND MAY BE TRANSFERRED ONLY IN BLOCKS
HAVING AN AGGREGATE PRINCIPAL AMOUNT OF NOT LESS THAN $100,000. ANY ATTEMPTED
TRANSFER OF SECURITIES IN A BLOCK HAVING AN AGGREGATE PRINCIPAL AMOUNT OF LESS
THAN $100,000 AND MULTIPLES OF $1,000 IN EXCESS THEREOF SHALL BE DEEMED TO BE
VOID AND OF NO LEGAL EFFECT WHATSOEVER. ANY SUCH PURPORTED TRANSFEREE SHALL BE
DEEMED NOT TO BE THE HOLDER OF SUCH SECURITIES FOR ANY PURPOSE, INCLUDING, BUT
NOT LIMITED TO, THE RECEIPT OF PRINCIPAL OF OR INTEREST ON SUCH SECURITIES, AND
SUCH PURPORTED TRANSFEREE SHALL BE DEEMED TO HAVE NO INTEREST WHATSOEVER IN SUCH
SECURITIES.

         THE HOLDER OF THIS SECURITY BY ITS ACCEPTANCE HEREOF ALSO AGREES,
REPRESENTS AND WARRANTS THAT IT IS NOT AN EMPLOYEE BENEFIT, INDIVIDUAL
RETIREMENT ACCOUNT OR OTHER PLAN OR ARRANGEMENT SUBJECT TO TITLE I OF THE
EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974, AS AMENDED ("ERISA"), OR
SECTION 4975 OF THE INTERNAL REVENUE CODE OF 1986, AS AMENDED (THE "CODE") (EACH
A "PLAN"), OR AN ENTITY WHOSE UNDERLYING ASSETS INCLUDE "PLAN ASSETS" BY REASON
OF ANY PLAN'S INVESTMENT IN THE ENTITY, AND NO PERSON INVESTING "PLAN ASSETS" OF
ANY PLAN MAY ACQUIRE OR HOLD THE SECURITIES OR ANY INTEREST THEREIN, UNLESS SUCH
PURCHASER OR HOLDER IS ELIGIBLE FOR THE EXEMPTIVE RELIEF AVAILABLE UNDER U.S.
DEPARTMENT OF LABOR PROHIBITED TRANSACTION CLASS EXEMPTION 96-23, 95-60, 91-38,
90-1 OR 84-14 OR ANOTHER APPLICABLE EXEMPTION OR ITS PURCHASE AND HOLDING OF
THIS


                                      A-1

<PAGE>   2


SECURITY IS NOT PROHIBITED BY SECTION 406 OF ERISA OR SECTION 4975 OF THE CODE
WITH RESPECT TO SUCH PURCHASE OR HOLDING. ANY PURCHASER OR HOLDER OF THE
SECURITIES OR ANY INTEREST THEREIN WILL BE DEEMED TO HAVE REPRESENTED BY ITS
PURCHASE AND HOLDING THEREOF THAT EITHER (i) IT IS NOT AN EMPLOYEE BENEFIT PLAN
WITHIN THE MEANING OF SECTION 3(3) OF ERISA, OR A PLAN TO WHICH SECTION 4975 OF
THE CODE IS APPLICABLE, A TRUSTEE OR OTHER PERSON ACTING ON BEHALF OF AN
EMPLOYEE BENEFIT PLAN OR PLAN, OR ANY OTHER PERSON OR ENTITY USING THE ASSETS OF
ANY EMPLOYEE BENEFIT PLAN OR PLAN TO FINANCE SUCH PURCHASE, OR (ii) SUCH
PURCHASE WILL NOT RESULT IN A PROHIBITED TRANSACTION UNDER SECTION 406 OF ERISA
OR SECTION 4975 OF THE CODE FOR WHICH THERE IS NO APPLICABLE STATUTORY OR
ADMINISTRATIVE EXEMPTION.


                                      A-2

<PAGE>   3


                       SUBORDINATED DEBENTURE CERTIFICATE
                              BANCFIRST OHIO CORP.

CUSIP No.: [_____________]
         $[_________________________]

            9.875% JUNIOR SUBORDINATED DEFERRABLE INTEREST DEBENTURE

                          DUE OCTOBER 15, 2029 Series B

         BancFirst Ohio Corp., an Ohio corporation (the "Corporation," which
term includes any successor Person under the Indenture hereinafter referred to),
for value received, hereby promises to pay to BFOH Capital Trust I or registered
assigns, the principal sum of $20,619,000 (Twenty Million Six Hundred Nineteen
Thousand Dollars) on October 15, 2029 (the "Maturity Date"), unless previously
prepaid, and to pay interest on the outstanding principal amount hereof from
October 18, 1999, or from the most recent interest payment date (each such date,
an "Interest Payment Date") to which interest has been paid or duly provided
for, semi-annually (subject to deferral as set forth herein) in arrears on April
15 and October 15 of each year, commencing April 15, 2000 at the rate of __% per
annum, until the principal hereof shall have become due and payable, and on any
overdue principal and premium, if any, and (without duplication and to the
extent that payment of such interest is enforceable under applicable law) on any
overdue installment of interest at the same rate per annum compounded
semi-annually ("Compounded Interest"). The amount of interest payable hereon
shall be computed on the basis of a 360-day year of twelve 30- day months. In
the event that any date on which the principal of (or premium, if any) or
interest on this Security is payable is not a Business Day (as defined in the
Indenture), then the payment payable on such date will be made on the next
succeeding day that is a Business Year, except that if such next succeeding
Business Day falls in the next succeeding calendar month such payment shall be
made on the immediately preceding Business Day (and without any interest or
other payment in respect of any such delay), with the same force and effect as
if made on such date. Pursuant to the Indenture, in certain circumstances the
Corporation will be required to pay Additional Sums (as defined in the
Indenture) with respect to this Security.

         The interest installment so payable, and punctually paid or duly
provided for, on any Interest Payment Date will, as provided in the Indenture,
be paid to the Person in whose name this Security (or one or more Predecessor
Securities, as defined in said Indenture) is registered at the close of business
on the regular record date for such interest installment, which shall be at the
close of business on the first day of the month in which the relevant Interest
Payment Date falls. Any such interest installment not punctually paid or duly
provided for shall forthwith cease to be payable to the holders on such regular
record date and may be paid to the Person in whose name this Security (or one or
more Predecessor Securities) is registered at the close of business on a special
record date to be fixed by the Debenture Trustee for the payment of such
defaulted interest, notice whereof shall be given to the holders of Securities
not less than 10 days prior to such special record date, or may be paid at any
time in any other lawful manner not inconsistent with the requirements of any
securities exchange on which the Securities may be listed, and upon such notice
as may be required by such exchange, all as more fully provided in the
Indenture.


                                      A-3


<PAGE>   4

         The principal of (and premium, if any) and interest (including
Compounded Interest and Additional Sums, if any) on this Security shall be
payable at the office or agency of the Debenture Trustee maintained for that
purpose in any coin or currency of the United States of America that at the time
of payment is legal tender for payment of public and private debts; PROVIDED,
HOWEVER, that payment of interest may be made at the option of the Corporation
by (i) check mailed to the holder at such address as shall appear in the
Security Register or (ii) transfer to an account maintained by the Person
entitled thereto, provided that proper written transfer instructions have been
received by the relevant record date. Notwithstanding the foregoing, so long as
the holder of this Security is the Property Trustee of BFOH Capital Trust I, the
payment of the principal of (and premium, if any) and interest (including
Compounded Interest and Additional Sums, if any) on this Security will be made
at such place and to such account as may be designated by such Property Trustee.

         The indebtedness evidenced by this Security is, to the extent provided
in the Indenture, subordinate and junior in right of payment to the prior
payment in full of all Senior Indebtedness, and this Security is issued subject
to the provisions of the Indenture with respect thereto. Each holder of this
Security, by accepting the same, (a) agrees to and shall be bound by such
provisions, (b) authorizes and directs the Debenture Trustee on his or her
behalf to take such action as may be necessary or appropriate to acknowledge or
effectuate the subordination so provided and (c) appoints the Debenture Trustee
his or her attorney-in-fact for any and all such purposes. Each holder hereof,
by his or her acceptance hereof, hereby waives all notice of the acceptance of
the subordination provisions contained herein and in the Indenture by each
holder of Senior Indebtedness, whether now outstanding or hereafter incurred,
and waives reliance by each such holder upon said provisions.

         This Security shall not be entitled to any benefit under the Indenture
or be valid or become obligatory for any purpose until the Certificate of
Authentication hereon shall have been signed by or on behalf of the Debenture
Trustee.

         The provisions of this Security are continued on the reverse side
hereof and such provisions shall for all purposes have the same effect as though
fully set forth at this place.


                                      A-4


<PAGE>   5


         IN WITNESS WHEREOF, the Corporation has caused this instrument to be
duly executed and sealed this [___] day of _________, 2000.


                                  BANCFIRST OHIO CORP.


                                  By:
                                      ----------------------------------------
                                      Gary N. Fields
                                      President and Chief Executive Officer


Attest:

By:
    ----------------------------------
    Name:
    Title:





                          CERTIFICATE OF AUTHENTICATION

     This is one of the 9.875% Junior Subordinated Deferrable Interest
Debentures Series B, of BANCFIRST OHIO CORP. referred to in the within-mentioned
Indenture.


                                  WILMINGTON TRUST COMPANY,
                                  not in its individual capacity but solely as
                                  Debenture Trustee


Dated:                            By:
      ---------------                -----------------------------------------
                                     Authorized Signatory



                                      A-5

<PAGE>   6

                          (FORM OF REVERSE OF SECURITY)

         This Security is one of the Securities of the Corporation (herein
sometimes referred to as the "Securities"), specified in the Indenture, all
issued or to be issued under and pursuant to an Indenture, dated as of October
18, 1999 (the "Indenture"), duly executed and delivered between the Corporation
and Wilmington Trust Company, as Debenture Trustee (the "Debenture Trustee"), to
which Indenture reference is hereby made for a description of the rights,
limitations of rights, obligations, duties and immunities thereunder of the
Debenture Trustee, the Corporation and the holders of the Securities.

         Upon the occurrence and continuation of a Special Event (as defined in
the Indenture) prior to October 15, 2009 (the "Initial Optional Redemption
Date"), the Corporation shall have the right, at any time within 90 days
following the occurrence of such Special Event, to prepay this Security in whole
(but not in part) at the Special Event Prepayment Price. "Special Event
Prepayment Price" shall mean, with respect to any prepayment of the Securities
following a Special Event, an amount in cash equal to the greater of (i) 100% of
the principal amount of the Securities to be prepaid, or (ii) the sum, as
determined by a Quotation Agent, of the present values of the remaining
scheduled payments of principal and interest on such Securities, discounted to
the prepayment date on a semi-annual basis (assuming a 360-day year consisting
of twelve 30-day months) at the Adjusted Treasury Rate, plus, in the case of
each of (i) and (ii), any accrued and unpaid interest (including Compounded
Interest and Additional Sums, if any) thereon to the date of such prepayment.

         In addition, the Corporation shall have the right to prepay this
Security, in whole or in part, at any time on or after the Initial Optional
Redemption Date (an "Optional Prepayment"), at the prepayment prices set forth
below plus, in each case, accrued and unpaid interest thereon (including
Compounded Interest and Additional Sums, if any) thereon to the applicable date
of prepayment (the "Optional Prepayment Price"), if prepaid during the 12-month
period beginning October 15, of the years indicated below.


          YEAR                            PERCENTAGE OF PRINCIPAL
          ----                            -----------------------
          2009                                    104.938%
          2010                                    104.444%
          2011                                    103.950%
          2012                                    103.457%
          2013                                    102.963%
          2014                                    102.469%
          2015                                    101.975%
          2016                                    101.481%
          2017                                    100.988%
          2018                                    100.494%
          2019 and thereafter                     100.000%

         The Prepayment Price shall be paid prior to 12:00 noon, New York City
time, on the date of such prepayment or at such earlier time as the Corporation
determines, PROVIDED that the Corporation shall deposit with the Debenture
Trustee an amount sufficient to pay the Prepayment


                                      A-6

<PAGE>   7

Price by 10:00 a.m., New York City time, on the date such Prepayment Price is to
be paid. Any prepayment pursuant to this paragraph will be made upon not less
than 30 days nor more than 60 days' prior written notice.

         If the Securities are only partially prepaid by the Corporation
pursuant to an Optional Prepayment, the particular Securities to be prepaid
shall be selected on a pro rata basis from the outstanding Securities not
previously called for prepayment; PROVIDED, HOWEVER, that with respect to
Securityholders that would be required to hold Securities with an aggregate
principal amount of less than $100,000 but more than an aggregate principal
amount of zero as a result of such pro rata prepayment, the Corporation shall
prepay Securities of each such Securityholder so that after such prepayment such
Securityholder shall hold Securities either with an aggregate principal amount
of at least $100,000 or such Securityholder no longer holds any Securities and
shall use such method (including, without limitation, by lot) as the Corporation
shall deem fair and appropriate; PROVIDED, FURTHER, that any such proration may
be made on the basis of the aggregate principal amount of Securities held by
each Securityholder thereof and may be made by making such adjustments as the
Corporation deems fair and appropriate in order that only Securities in
denominations of $1,000 or integral multiples thereof shall be prepaid. In the
event of prepayment of this Security in part only, a new Security or Securities
for the portion hereof that has not been prepaid will be issued in the name of
the holder hereof upon the cancellation hereof.

         Notwithstanding the foregoing, any prepayment of Securities by the
Corporation shall be subject to the receipt of any and all required regulatory
approvals.

         In case an Event of Default (as defined in the Indenture) shall have
occurred and be continuing, the principal of all of the Securities may be
declared, and upon such declaration shall become, due and payable, in the
manner, with the effect and subject to the conditions provided in the Indenture.

         The Indenture contains provisions permitting the Corporation and the
Debenture Trustee, with the consent of the holders of a majority in aggregate
principal amount of the Securities at the time outstanding (as defined in the
Indenture), to execute supplemental indentures for the purpose of adding any
provisions to or changing in any manner or eliminating any of the provisions of
the Indenture or of modifying in any manner the rights of the holders of the
Securities; PROVIDED, HOWEVER, that no such supplemental indenture shall,
without the consent of each holder of Securities then outstanding and affected
thereby, (i) change the Maturity Date of any Security, or reduce the rate or
extend the time of payment of interest thereon (subject to Article XVI of the
Indenture), or reduce the principal amount thereof, or change any of the
prepayment provisions or make the principal thereof or any interest or premium
thereon payable in any coin or currency other than U.S. dollars, or impair or
affect the right of any holder of Securities to institute suit for payment
thereof, or (ii) reduce the aforesaid percentage of Securities the holders of
which are required to consent to any such supplemental indenture. The Indenture
also contains provisions permitting the holders of a majority in aggregate
principal amount of the Securities at the time outstanding affected thereby, on
behalf of all of the holders of the Securities, to waive any past default in the
performance of any of the covenants contained in the Indenture, or established
pursuant to the Indenture, and its consequences, except a default in the payment
of the principal of or premium, if any, or interest on any of the Securities or
a default in respect of any covenant or provision under which the Indenture
cannot be modified or


                                      A-7


<PAGE>   8

amended without the consent of each holder of Securities then outstanding. Any
such consent or waiver by the holder of this Security (unless revoked as
provided in the Indenture) shall be conclusive and binding upon such holder and
upon all future holders and owners of this Security and of any Security issued
in exchange herefor or in place hereof (whether by registration of transfer or
otherwise), irrespective of whether or not any notation of such consent or
waiver is made upon this Security.

         No reference herein to the Indenture and no provision of this Security
or of the Indenture shall alter or impair the obligation of the Corporation,
which is absolute and unconditional, to pay the principal of (and premium, if
any) and interest (including Compounded Interest and Additional Sums, if any) on
this Security at the time and place and at the rate and in the money herein
prescribed.

         So long as no Event of Default shall have occurred and be continuing,
the Corporation shall have the right, at any time and from time to time during
the term of the Securities, to defer payments of interest by extending the
interest payment period (an "Extended Interest Payment Period") of such
Securities for a period not (i) exceeding 10 consecutive semi-annual periods,
including the first such semi-annual period during such extension period, (ii)
extending beyond the Maturity Date of the Securities or (iii) ending on a date
other than an Interest Payment Date, at the end of which period the Corporation
shall pay all interest then accrued and unpaid (together with interest thereon
at the rate specified for the Securities to the extent that payment of such
interest is enforceable under applicable law). Before the termination of any
such Extended Interest Payment Period, the Corporation may further defer
payments of interest by further extending such Extended Interest Payment Period,
PROVIDED that such Extended Interest Payment Period, together with all such
previous and further extensions within such Extended Interest Payment Period,
(i) shall not exceed 10 consecutive semi-annual periods including the first
semi-annual period during such Extended Interest Payment Period, (ii) shall not
end on any date other than an Interest Payment Date, and (iii) shall not extend
beyond the Maturity Date of the Securities. Upon the termination of any such
Extended Interest Payment Period and the payment of all accrued and unpaid
interest and any additional amounts then due, the Corporation may commence a new
Extended Interest Payment Period, subject to the foregoing requirements. No
interest shall be due and payable during an Extended Interest Payment Period,
except at the end thereof, but the Corporation may prepay at any time all or any
portion of the interest accrued during an Extended Interest Payment Period.

         The Corporation has agreed that it will not (i) declare or pay any
dividends or distributions on, or redeem, purchase, acquire, or make a
liquidation payment with respect to, any of the Corporation's capital stock,
(ii) make any payment of principal of or interest or premium, if any, on or
repay, repurchase or redeem any debt securities (including other Debentures) of
the Corporation that rank pari passu with or junior in right of payment to the
Securities or (iii) make any guarantee payments with respect to any guarantee by
the Corporation of the debt securities of any Subsidiary of the Corporation
(including Other Guarantees) if such guarantee ranks pari passu with or junior
in right of payment to the Securities (other than (a) dividends or distributions
of Corporation's capital stock (which includes Common Stock and preferred
stock), (b) any declaration of a dividend in connection with the implementation
of a stockholders' rights plan, or the issuance of stock under any such plan in
the future, or the redemption or repurchase of any such rights pursuant thereto,
(c) payments under the Capital Securities Guarantee, as defined in the
Indenture, (d) as a result of a reclassification of the


                                      A-8

<PAGE>   9

Corporation's capital stock or the exchange or conversion of one class or series
of the Corporation's capital stock for another class or series of the
Corporation's capital stock, (e) the purchase of fractional interests in shares
of the Corporation's capital stock pursuant to the conversion or exchange
provisions of such capital stock or the security being converted or exchanged,
and (f) purchases of Common Stock related to the issuance of Common Stock or
rights under any of the Corporation's benefit or compensation plans for its
directors, officers or employees or any of the Corporation's dividend
reinvestment plans), if at such time (1) there shall have occurred any event of
which the Corporation has actual knowledge that (a) is, or with the giving of
notice or the lapse of time, or both, would constitute, an Event of Default and
(b) in respect of which the Corporation shall not have taken reasonable steps to
cure, (2) if such Securities are held by the Property Trustee of BFOH Capital
Trust I, the Corporation shall be in default with respect to its payment of any
obligations under the Capital Securities Guarantee or (3) the Corporation shall
have given notice of its election to exercise its right to commence an Extended
Interest Payment Period, and shall not have rescinded such Notice, and such
Extended Interest Payment Period or any extension thereof shall have commenced
and be continuing.

         Subject to (i) the Corporation having received any required regulatory
approvals and (ii) the Administrative Trustees of BFOH Capital Trust I having
received an opinion of counsel to the effect that such distribution will not
cause the holders of Capital Securities to recognize gain or loss for federal
income tax purposes, the Corporation will have the right at any time to
liquidate the Trust and, after satisfaction of liabilities of creditors of the
Trust as required by applicable law, to cause the Securities to be distributed
to the holders of the Trust Securities in liquidation of the Trust.

         The Securities are issuable only in registered form without coupons in
minimum denominations of $100,000 and multiples of $1,000 in excess thereof. As
provided in the Indenture and subject to the transfer restrictions limitations
as may be contained herein and therein from time to time, this Security is
transferable by the holder hereof on the Security Register of the Corporation,
upon surrender of this Security for registration of transfer at the office or
agency of the Corporation in Wilmington, Delaware accompanied by a written
instrument or instruments of transfer in form satisfactory to the Corporation or
the Debenture Trustee duly executed by the holder hereof or his or her attorney
duly authorized in writing, and thereupon one or more new Securities of
authorized denominations and for the same aggregate principal amount will be
issued to the designated transferee or transferees. No service charge will be
made for any such registration of transfer, but the Corporation may require
payment of a sum sufficient to cover any tax or other governmental charge
payable in relation thereto.

         Prior to due presentment for registration of transfer of this Security,
the Corporation, the Debenture Trustee, any authenticating agent, any paying
agent, any transfer agent and the security registrar may deem and treat the
holder hereof as the absolute owner hereof (whether or not this Security shall
be overdue and notwithstanding any notice of ownership or writing hereon made by
anyone other than the security registrar for the Securities) for the purpose of
receiving payment of or on account of the principal hereof and premium, if any,
and (subject to the Indenture) interest due hereon and for all other purposes,
and neither the Corporation nor the Debenture Trustee nor any authenticating
agent nor any paying agent nor any transfer agent nor any security registrar
shall be affected by any notice to the contrary.


                                      A-9


<PAGE>   10


         No recourse shall be had for the payment of the principal of or
premium, if any, or interest (including Compounded Interest and Additional Sums,
if any) on this Security, or for any claim based hereon, or otherwise in respect
hereof, or based on or in respect of the Indenture, against any incorporator,
stockholder, employee, officer or director, past, present or future, as such, of
the Corporation or of any predecessor or successor Person, whether by virtue of
any constitution, statute or rule of law, or by the enforcement of any
assessment or penalty or otherwise, all such liability being, by the acceptance
hereof and as part of the consideration for the issuance hereof, expressly
waived and released.

         All terms used in this Security that are defined in the Indenture shall
have the meanings assigned to them in the Indenture.

         THE INDENTURE AND THE SECURITIES SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO CONFLICT OF
LAW PRINCIPLES THEREOF.