EXHIBIT 99.2
 
                         NOTICE OF GUARANTEED DELIVERY
                                 FOR TENDER OF
                       9.25% ORIGINAL CAPITAL SECURITIES
                (LIQUIDATION AMOUNT $1,000 PER CAPITAL SECURITY)
 
                                       OF
 
                             ONBANK CAPITAL TRUST I
                    FULLY AND UNCONDITIONALLY GUARANTEED BY
 
                                ONBANCORP. INC.
 
    This Notice of Guaranteed Delivery, or one substantially equivalent to this
form, must be used to accept the Exchange Offer (as defined below) if (i)
certificates for the Trust's (as defined below) 9.25% Series A Capital
Securities (the "Original Capital Securities") are not immediately available,
(ii) Original Capital Securities, the Letter of Transmittal and all other
required documents cannot be delivered to The Bank of New York (the "Exchange
Agent") on or prior to the Expiration Date (as defined in the Prospectus
referred to below) or (iii) the procedures for delivery by book-entry transfer
cannot be completed on a timely basis. This Notice of Guaranteed Delivery may be
delivered by hand, overnight courier or mail, or transmitted by facsimile
transmission, to the Exchange Agent. See "The Exchange Offer-Procedures for
Tendering Original Capital Securities" in the Prospectus. In addition, in order
to utilize the guaranteed delivery procedure to tender Original Capital
Securities pursuant to the Exchange Offer, a completed, signed and dated Letter
of Transmittal relating to the Original Capital Securities (or facsimile
thereof) must also be received by the Exchange Agent on or prior to the
Expiration Date. Capitalized terms not defined herein have the meanings assigned
to them in the Prospectus.
 
                 THE EXCHANGE AGENT FOR THE EXCHANGE OFFER IS:
                              THE BANK OF NEW YORK
 

                                                            
           BY MAIL:                 FACSIMILE TRANSMISSIONS:      BY HAND OR OVERNIGHT DELIVERY:
 (Registered or Certified Mail    (ELIGIBLE INSTITUTIONS ONLY)         The Bank of New York
         recommended)                    (212) 571-3080                 101 Barclay Street
     The Bank of New York        TO CONFIRM BY TELEPHONE OR FOR   Corporate Trust Services Window
    101 Barclay Street, 7E              INFORMATION CALL:                  Ground Level
   New York, New York 10286              (212) 815-6337              New York, New York 10286
   Attention: Reorganization                                         Attention: Reorganization
            Section                                                           Section
          Odell Romeo                                                       Odell Romeo

 
    DELIVERY OF THIS NOTICE OF GUARANTEED DELIVERY TO AN ADDRESS OTHER THAN AS
SET FORTH ABOVE OR TRANSMISSION OF THIS NOTICE OF GUARANTEED DELIVERY VIA
FACSIMILE TO A NUMBER OTHER THAN AS SET FORTH ABOVE WILL NOT CONSTITUTE A VALID
DELIVERY.
 
    This Notice of Guaranteed Delivery is not to be used to guarantee
signatures. If a signature on a Letter of Transmittal is required to be
guaranteed by an "Eligible Institution" under the instructions thereto, such
signature guarantee must appear in the applicable space provided in the
signature box on the Letter of Transmittal.

Ladies and Gentlemen:
 
    The undersigned hereby tenders to OnBank Capital Trust I, a Delaware
business trust (the "Trust") and to ONBANCorp, Inc., a Delaware corporation (the
"Company"), upon the terms and subject to the conditions set forth in the
Prospectus dated           , 1997 (as the same may be amended or supplemented
from time to time, the "Prospectus"), and the related Letter of Transmittal
(which together constitute the "Exchange Offer"), receipt of which is hereby
acknowledged, the aggregate principal amount of Original Capital Securities set
forth below pursuant to the guaranteed delivery procedures set forth in the
Prospectus under the caption "The Exchange Offer--Procedures for Tendering
Original Capital Securities."
Aggregate Liquidation Amount ___________________________________________________
Name(s) of Registered Holder(s): _______________________________________________
Amount Tendered: $(1) __________________________________________________________
Certificate No.(s) (if available): _____________________________________________
 
(Total Liquidation Amount Represented by Original Capital Securities
Certificate(s)
$ ________________
 
If Original Capital Securities will be tendered by book-entry transfer, provide
the following information:
DTC Account Number: ____________________________________________________________
Date: __________________________________________________________________________
 
- ------------------------
 
(1)   Must be in denominations of a Liquidation Amount of $1,000 and any
    integral multiple thereof, and not less than $100,000 aggregate Liquidation
    Amount.
 
    All authority herein conferred or agreed to be conferred shall survive the
    death or incapacity of the undersigned and every obligation of the
    undersigned hereunder shall be binding upon the heirs, personal
    representatives, successors and assigns of the undersigned.
 
                                       2

 
                                PLEASE SIGN HERE
    X ______________________________________________________________________
    X ______________________________________________________________________
                SIGNATURE(S) OF OWNER(S) OR AUTHORIZED SIGNATORY
    Date ___________________________________________________________________
    Area Code and Telephone Number: ________________________________________
 
        Must be signed by the holder(s) of the Original Capital Securities
    as their name(s) appear(s) on certificates for Original Capital
    Securities or on a security position listing, or by person(s) authorized
    to become registered holder(s) by endorsement and documents transmitted
    with this Notice of Guaranteed Delivery. If signature is by a trustee,
    executor, administrator, guardian, attorney-in-fact, officer or other
    person acting in a fiduciary or representative capacity, such person
    must set forth his or her full title below. Please print name(s) and
    address(es)
    (Name(s): ______________________________________________________________
    Capacity: ______________________________________________________________
    Address(es): ___________________________________________________________
     _______________________________________________________________________
 
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                                   GUARANTEE
                    (NOT TO BE USED FOR SIGNATURE GUARANTEE)
 
        The undersigned, a firm or other entity identified in Rule 17Ad-15
    under the Securities Exchange Act of 1934, as amended, as an "eligible
    guarantor institution," including (as such terms are defined therein):
    (i) a bank; (ii) a broker, dealer, municipal securities broker,
    municipal securities dealer, government securities broker or government
    securities dealer; (iii) a credit union; (iv) a national securities
    exchange, registered securities association or clearing agency; or (v) a
    savings association that is a participant in a Securities Transfer
    Association recognized program (each of the foregoing being referred to
    as an "Eligible Institution"), hereby guarantees to deliver to the
    Exchange Agent, at one of its addresses set forth above, either the
    Original Capital Securities tendered hereby in proper form for transfer,
    or confirmation of the book-entry transfer of such Original Capital
    Securities to the Exchange Agent's account at The Depository Trust
    Company ("DTC"), pursuant to the procedures for book-entry transfer set
    forth in the Prospectus, in either case together with one or more
    properly completed and duly executed Letter(s) of Transmittal (or
    facsimile thereof) and any other required documents within three
    business days after the date of execution of this Notice of Guaranteed
    Delivery.
 
        The undersigned acknowledges that it must deliver the Letter(s) of
    Transmittal and the Original Capital Securities tendered hereby to the
    Exchange Agent within the time period set forth above and that failure
    to do so could result in a financial loss to the undersigned.
 
                             (Please Type or Print)
    Name of Firm: __________________________________________________________
    Address: _______________________________________________________________
    Zip Code _______________________________________________________________
    Area Code and Telephone No. ____________________________________________
    Authorized Signature ___________________________________________________
    Title: _________________________________________________________________
    Dated: _________________________________________________________________
 
        NOTE: DO NOT SEND CERTIFICATES FOR ORIGINAL CAPITAL SECURITIES WITH
              THIS FORM. CERTIFICATES FOR ORIGINAL CAPITAL SECURITIES SHOULD
              ONLY BE SENT WITH YOUR LETTER OF TRANSMITTAL.
 
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