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                                                                    EXHIBIT 4.18


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                      COMMON SECURITIES GUARANTEE AGREEMENT

                                 ONBANCorp, Inc.

                          Dated as of February 4, 1997

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                                TABLE OF CONTENTS

                                                           PAGE

                                    ARTICLE I
                         DEFINITIONS AND INTERPRETATION

SECTION 1.1. Definitions Interpretation  . . . . . . . . . 2

                                   ARTICLE II
                                    GUARANTEE

SECTION 2.1. Guarantee . . . . . . . . . . . . . . . . . . 2
SECTION 2.2. Waiver of Notice and Demand . . . . . . . . . 2
SECTION 2.3. Obligations Not Affected  . . . . . . . . . . 2
SECTION 2.4. Rights of Holders . . . . . . . . . . . . . . 4
SECTION 2.5. Guarantee of Payment  . . . . . . . . . . . . 5
SECTION 2.6. Subrogation . . . . . . . . . . . . . . . . . 5
SECTION 2.7. Independent Obligations . . . . . . . . . . . 5

                                   ARTICLE III
                    LIMITATION OF TRANSACTIONS; SUBORDINATION

SECTION 3.1. Limitation of Transactions  . . . . . . . . . 5
SECTION 3.2. Ranking . . . . . . . . . . . . . . . . . . . 6

                                    ARTICLE IV
                                   TERMINATION

SECTION 4.1. Termination . . . . . . . . . . . . . . . . . 6

                                    ARTICLE V
                                  MISCELLANEOUS

SECTION 5.1. Successors and Assigns  . . . . . . . . . . . 7
SECTION 5.2. Amendments  . . . . . . . . . . . . . . . . . 7
SECTION 5.3. Notices . . . . . . . . . . . . . . . . . . . 7
SECTION 5.4. Benefit . . . . . . . . . . . . . . . . . . . 8
SECTION 5.5. Governing Law . . . . . . . . . . . . . . . . 8


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                      COMMON SECURITIES GUARANTEE AGREEMENT

         This GUARANTEE AGREEMENT (the "Common Securities Guarantee"), dated as
of February 4, 1997, is executed and delivered by ONBANCorp, Inc., a Delaware
corporation (the "Guarantor"), for the benefit of the Holders (as defined
herein) from time to time of the Common Securities (as defined herein) of OnBank
Capital Trust I, a Delaware business trust (the "Issuer").

         WHEREAS, pursuant to an Amended and Restated Declaration of Trust (the
"Declaration"), dated as of February 4, 1997, among the Trustees of the Issuer
named therein, the Guarantor, as sponsor, and the holders from time to time of
undivided beneficial interests in the assets of the Issuer, the Issuer is
issuing on the date hereof 1,856 common securities designated the 9.25% Common
Securities (the "Common Securities"), having an aggregate stated liquidation
amount of $1,856,000;

         WHEREAS, as incentive for the Holders to purchase the Common
Securities, the Guarantor desires to irrevocably and unconditionally agree, to
the extent set forth in this Common Securities Guarantee, to pay to the Holders
the Guarantee Payments (as defined herein) and to make certain other payments on
the terms and conditions set forth herein; and

         WHEREAS, the Guarantor is also executing and delivering a guarantee
agreement (the "Series A Capital Securities Guarantee") for the benefit of the
holders of the Series A Capital Securities (as defined in the Declaration) and
upon consummation of the Exchange Offer (as defined in the Declaration) will
execute and deliver a guarantee agreement (the "Series B Capital Securities
Guarantee") for the benefit of the holders of the Series B Capital Securities
(as defined in the Declaration), each in substantially identical terms to this
Common Securities Guarantee, except that if an Event of Default (as defined in
the Declaration) has occurred and is continuing, the rights of Holders to
receive Guarantee Payments under this Common Securities Guarantee are
subordinated to the rights of holders of Capital Securities to receive Guarantee
Payments under the Series A Capital Securities Guarantee and the Series B
Capital Securities Guarantee, as the case may be.

         NOW, THEREFORE, in consideration of the purchase by each Holder, which
purchase the Guarantor hereby acknowledges shall benefit the Guarantor, the
Guarantor executes and delivers this Common Securities Guarantee for the benefit
of the Holders.


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                                    ARTICLE I
                         DEFINITIONS AND INTERPRETATION

SECTION 1.1.   DEFINITIONS INTERPRETATION

        In this Common Securities Guarantee, unless the context otherwise
requires:

         (a) Capitalized terms used in this Common Securities Guarantee but not
     defined in the preamble above have the respective meanings assigned to them
     in this Section 1.1;

         (b) Terms defined in the Declaration as at the date of execution of
     this Common Securities Guarantee have the same meaning when used in this
     Common Securities Guarantee unless otherwise defined in this Common
     Securities Guarantee;

         (c) a term defined anywhere in this Common Securities Guarantee has the
     same meaning throughout;

         (d) all references to "the Common Securities Guarantee" or "this Common
     Securities Guarantee" are to this Common Securities Guarantee as modified,
     supplemented or amended from time to time;

         (e) all references in this Common Securities Guarantee to Articles and
     Sections are to Articles and Sections of this Common Securities Guarantee
     unless otherwise specified; and

         (f) a reference to the singular includes the plural and vice versa.

         "Guarantee Payments" means the following payments or distributions,
without duplication, with respect to the Common Securities, to the extent not
paid or made by the Issuer: (i) any accrued and unpaid Distributions that are
required to be paid on such Common Securities to the extent the Issuer has funds
on hand legally available therefor at such time, (ii) the redemption price,
including all accrued and unpaid Distributions to the date of redemption (the
"Redemption Price") to the extent the Issuer has funds on hand legally available
therefor at such time, with respect to any Common Securities called for
redemption by the Issuer, and (iii) upon a voluntary or involuntary termination
and liquidation of the Issuer (other than in connection with the distribution of
Debentures to the Holders in exchange for Common Securities as provided in the
Declaration), the lesser of (a) the aggregate of the liquidation amount and all
accumulated and unpaid Distributions on the Common Securities to the date of
payment, to the extent the Issuer has funds on hand legally available therefor,
and (b) the amount of assets of the Issuer remaining available for distribution
to Holders in liquidation of the

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Issuer (in either case, the "Liquidation Distribution"). If an Event of Default
has occurred and is continuing, no Guarantee Payments with respect to the Common
Securities shall be made until holders of Capital Securities shall be paid in
full the Guarantee Payments to which they are entitled under the Series A
Capital Securities Guarantee and the Series B Capital Securities Guarantee.

         "Holder" means any holder, as registered on the books and records
of the Issuer, of any Common Securities.

         "Other Guarantees" means all guarantees to be issued by the Guarantor
with respect to common securities (if any) similar to the Common Securities
issued by other trusts to be established by the Guarantor (if any), in each case
similar to the Issuer.

                                   ARTICLE II
                                   GUARANTEE

SECTION 2.1.   GUARANTEE

         The Guarantor irrevocably and unconditionally agrees to pay in full to
the Holders the Guarantee Payments (without duplication of amounts theretofore
paid by the Issuer), as and when due, regardless of any defense, right of
set-off or counter-claim which the Issuer may have or assert. The Guarantor's
obligation to make a Guarantee Payment may be satisfied by direct payment of the
required amounts by the Guarantor to the Holders or by causing the Issuer to pay
such amounts to the Holders.

SECTION 2.2.  WAIVER OF NOTICE AND DEMAND

         The Guarantor hereby waives notice of acceptance of this Common
Securities Guarantee and of any liability to which it applies or may apply,
presentment, demand for payment, any right to require a proceeding first against
the Issuer or any other Person before proceeding against the Guarantor, protest,
notice of nonpayment, notice of dishonor, notice of redemption and all other
notices and demands.

SECTION 2.3.  OBLIGATIONS NOT AFFECTED

         The obligations, covenants, agreements and duties of the Guarantor
under this Common Securities Guarantee shall in no way be affected or impaired
by reason of the happening from time to time of any of the following:

         (a) the release or waiver, by operation of law or otherwise, of the
     performance or observance by the Issuer of

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     any express or implied agreement, covenant, term or condition relating to
     the Common Securities to be performed or observed by the Issuer;

         (b) the extension of time for the payment by the Issuer of all or any
     portion of the Distributions, Redemption Price, Liquidation Distribution or
     any other sums payable under the terms of the Common Securities or the
     extension of time for the performance of any other obligation under,
     arising out of, or in connection with, the Common Securities (other than an
     extension of time for payment of Distributions, Redemption Price,
     Liquidation Distribution or other sum payable that results from the
     extension of any interest payment period on the Debentures permitted by the
     Indenture);

         (c) any failure, omission, delay or lack of diligence on the part of
     the Holders to enforce, assert or exercise any right, privilege, power or
     remedy conferred on the Holders pursuant to the terms of the Common
     Securities, or any action on the part of the Issuer granting indulgence or
     extension of any kind;

         (d) the voluntary or involuntary liquidation, dissolution, sale of any
     collateral, receivership, insolvency, bankruptcy, assignment for the
     benefit of creditors, reorganization, arrangement, composition or
     readjustment of debt of, or other similar proceedings affecting, the Issuer
     or any of the assets of the Issuer;

         (e) any invalidity of, or defect or deficiency in, the Common
     Securities;

         (f) the settlement or compromise of any obligation guaranteed hereby or
     hereby incurred; or

         (g) any other circumstance whatsoever that might otherwise constitute a
     legal or equitable discharge or defense of a guarantor, it being the intent
     of this Section 2.3 that the obligations of the Guarantor with respect to
     the Guarantee Payments shall be absolute and unconditional under any and
     all circumstances.

There shall be no obligation of the Holders to give notice to, or obtain consent
of, the Guarantor with respect to the happening of any of the foregoing.

SECTION 2.4.   RIGHTS OF HOLDERS

         The Guarantor expressly acknowledges that any Holder may institute
a legal proceeding directly against the Guarantor to enforce its rights under
this Common Securities Guarantee,

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without first instituting a legal proceeding against the Issuer or any other
Person.

SECTION 2.5.  GUARANTEE OF PAYMENT

         This Common Securities Guarantee creates a guarantee of payment and not
of collection.

SECTION 2.6.  SUBROGATION

         The Guarantor shall be subrogated to all (if any) rights of the Holders
against the Issuer in respect of any amounts paid to such Holders by the
Guarantor under this Common Securities Guarantee; PROVIDED, HOWEVER, that the
Guarantor shall not (except to the extent required by mandatory provisions of
law) be entitled to enforce or exercise any rights which it may acquire by way
of subrogation or any indemnity, reimbursement or other agreement, in all cases
as a result of payment under this Common Securities Guarantee, if, at the time
of any such payment, any amounts are due and unpaid under this Common Securities
Guarantee. If any amount shall be paid to the Guarantor in violation of the
preceding sentence, the Guarantor agrees to hold such amount in trust for the
Holders and to pay over such amount to the Holders.

SECTION 2.7.  INDEPENDENT OBLIGATIONS

         The Guarantor acknowledges that its obligations hereunder are
independent of the obligations of the Issuer with respect to the Common
Securities and that the Guarantor shall be liable as principal and as debtor
hereunder to make Guarantee Payments pursuant to the terms of this Common
Securities Guarantee notwithstanding the occurrence of any event referred to in
subsections (a) through (g), inclusive, of Section 2.3 hereof.

                                   ARTICLE III
                    LIMITATION OF TRANSACTIONS; SUBORDINATION

SECTION 3.1.  LIMITATION OF TRANSACTIONS

         So long as any Common Securities remain outstanding, the Guarantor 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 Guarantor's
capital stock (which includes common stock and preferred stock) or (ii) make any
payment of principal of, or premium, if any, or interest on or repay, repurchase
or redeem any debt securities of the Guarantor (including Other Debentures) that
rank PARI PASSU with or junior in right of payment to the Debentures or (iii)
make any guarantee payments with respect to any guarantee by the Guarantor of
the debt securities of any subsidiary of the

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Guarantor (including under Other Guarantees) if such guarantee ranks PARI
PASSU with or junior in right of payment to the Debentures (other than (a)
dividends or distributions in shares of, or options, warrants or rights to
subscribe for or purchase shares of, common stock of the Guarantor, (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, (d) as a result
of a reclassification of the Guarantor's capital stock or the exchange or the
conversion of one class or series of the Guarantor's capital stock for
another class or series of the Guarantor's capital stock, (e) the purchase of
fractional interests in shares of the Guarantor'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 Guarantor's benefit
plans for its directors, officers or employees or any of the Guarantor's
dividend reinvestment plans) if at such time (i) there shall have occurred
any event of which the Guarantor has actual knowledge that (a) is, or with
the giving of notice or the lapse of time, or both, would be, an Event of
Default and (b) in respect of which the Guarantor shall not have taken
reasonable steps to cure, (ii) if such Debentures are held by the Property
Trustee, the Guarantor shall be in default with respect to its payment of any
obligations under the Capital Securities Guarantee or (iii) the Guarantor
shall have given notice of its election of the exercise of its right to
extend the interest payment period pursuant to Section 16.01 of the Indenture
and any such extension shall be continuing.

SECTION 3.2.  RANKING

         This Common Securities Guarantee will constitute an unsecured
obligation of the Guarantor and will rank (i) subordinate and junior in right of
payment to Senior indebtedness (as defined in the Indenture), to the same extent
and in the same manner that the Debentures are subordinated to Senior
Indebtedness pursuant to the Indenture, (ii) PARI PASSU with the Debentures, the
Other Debentures and with any Other Guarantee, and (iii) senior to the
Guarantor's capital stock.

                                   ARTICLE IV
                                   TERMINATION

SECTION 4.1.  TERMINATION

         This Common Securities Guarantee shall terminate (i) upon full payment
of the Redemption Price of all Common Securities, (ii) upon the distribution of
all of the Debentures to all

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the Holders and the holders of the Capital Securities or (iii) upon full payment
of the amounts payable in accordance with the Declaration upon liquidation of
the Issuer. Notwithstanding the foregoing, this Common Securities Guarantee will
continue to be effective or will be reinstated, as the case may be, if at any
time any Holder must restore payment of any sums paid under the Common
Securities or under this Common Securities Guarantee.

                                    ARTICLE V
                                  MISCELLANEOUS

SECTION 5.1.  SUCCESSORS AND ASSIGNS

         All guarantees and agreements contained in this Common Securities
Guarantee shall bind the successors, assigns, receivers, trustees and
representatives of the Guarantor and shall inure to the benefit of the Holders
then outstanding.

SECTION 5.2.  AMENDMENTS

         Except with respect to any changes which do not adversely affect the
rights of Holders (in which case no consent of Holders will be required), this
Common Securities Guarantee may only be amended with the prior approval of the
Holders of at least a majority in liquidation amount of all the outstanding
Common Securities. The provisions of Section 12.2 of the Declaration with
respect to meetings of holders of the Securities apply to the giving of such
approval.

SECTION 5.3.  NOTICES

         All notices provided for in this Common Securities Guarantee shall be
in writing, duly signed by the party giving such notice, and shall be delivered,
telecopied or mailed by registered or certified mail, as follows:

         (a) if given to the Issuer, in care of the Administrative Trustee at
     the Issuer's mailing address set forth below (or such other address as the
     Issuer may give notice of to the Holders):

                             OnBank Capital Trust I
                             c/o ONBANCorp, Inc.
                             101 South Salina Street
                             P.O. Box 4983
                             Syracuse, New York 13221-4983
                             Attention: William M. LeBeau
                                        Administrative Trustee
                             Telecopy: (315) 424-5951

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         (b) if given to the Guarantor, at the Guarantor's mailing address set
     forth below (or such other address as the Guarantor may give notice to the
     Holders):

            ONBANCorp, Inc.
            101 South Salina Street
            Syracuse, New York 13221-4983
            Attention: Robert J. Berger
            Telecopy: (315) 424-5951

         (c) if given to any Holder, at the address set forth on the books and
     records of the Issuer.

         All such notices shall be deemed to have been given when received in
person, telecopied with receipt confirmed, or mailed by first class mail,
postage prepaid except that if a notice or other document is refused delivery or
cannot be delivered because of a changed address of which no notice was given,
such notice or other document shall be deemed to have been delivered on the date
of such refusal or inability to deliver.

SECTION 5.4.  BENEFIT

         This Common Securities Guarantee is solely for the benefit of the
Holders and is not separately transferable from the Common Securities.

SECTION 5.5.  GOVERNING LAW

         THIS COMMON SECURITIES GUARANTEE SHALL BE GOVERNED BY, AND CONSTRUED
AND INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK, WITHOUT
REGARD TO CONFLICTS OF LAWS PRINCIPLES THEREOF.

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         THIS COMMON SECURITIES GUARANTEE is executed as of the day and year
first above written.

                                       ONBANCORP, INC.


                                       By: /s/ ROBERT J. BERGER
                                          -------------------------------
                                          Name: Robert J. Berger
                                          Title: Senior Vice President,
                                          Treasurer & Chief
                                          Financial Officer