EXHIBIT 5.1

                    SKADDEN, ARPS, SLATE, MEAGHER & FLOM LLP
                                919 THIRD AVENUE
                               NEW YORK 10022-3897

                                   ----------

                               TEL: (212) 735-3000
                               FAX: (212) 735-2000


                                  June 23, 1997


OnBank Capital Trust I
ONBANCorp, Inc.
c/o   ONBANCorp, Inc.
      101 South Salina Street
      Syracuse, New York  13202

Ladies and Gentlemen:

            We have acted as special counsel to ONBANCorp, Inc., a Delaware bank
holding company (the "Company"), and OnBank Capital Trust I, a business trust
formed under the Business Trust Act of the State of Delaware (Chapter 38, Title
12, of the Delaware Code, 12 Del. C. ss.ss. 3801 et. seq.) (the "Trust"), in
connection with the preparation of the Registration Statement on Form S-4 (the
"Registration Statement") to be filed by the Company and the Trust with the
Securities and Exchange Commission (the "Commission") on the date hereof. The
Registration Statement relates to the registration under the Securities Act of
1933, as amended (the "Act"), of $60,000,000 aggregate liquidation amount of
9.25% Exchange Capital Securities (the "Capital Securities") in connection with
a proposed exchange offer (the "Exchange Offer").

            The Capital Securities are to be issued pursuant to the Amended and
Restated Declaration of Trust of the Trust, dated as of February 4, 1997 (the
"Declaration"), among Donald G. Cook, William LeBeau and Randy J. Wiley, as
administrative trustees, The Bank of New York, as property trustee (the
"Property Trustee"), The Bank of New York (Delaware), as Delaware trustee and
the Company, as sponsor.


OnBank Capital Trust I 
ONBANCorp, Inc.
June 23, 1997
Page 2


            This opinion is being furnished in accordance with the requirements
of Item 601(b)(5) of Regulation S-K under the Act.

            In connection with this opinion, we have examined originals or
copies, certified or otherwise identified to our satisfaction, of (i) the
Registration Statement; (ii) an executed copy of the Registration Rights
Agreement, dated as of February 4, 1997 (the "Registration Rights Agreement"),
among the Company, the Trust and Keefe, Bruyette & Woods, Inc., Blaylock &
Partners, L.P., and Sandler O'Neill & Partners, L.P.; (iii) the form of the
Capital Securities and a specimen certificate thereof; (iv) the Certificate of
Incorporation and Bylaws of the Company; (v) the Certificate of Trust of the
Trust filed with the Secretary of State of the State of Delaware on January 24,
1997; and (vi) the Declaration. We have also examined originals or copies,
certified or otherwise identified to our satisfaction, of such other documents,
certificates and records as we have deemed necessary or appropriate as a basis
for the opinions set forth herein.

            In our examination, we have assumed the legal capacity of all
natural persons, the genuineness of all signatures, the authenticity of all
documents submitted to us as originals, the conformity to original documents of
all documents submitted to us as certified or photostatic copies and the
authenticity of the originals of such latter documents. In making our
examination of documents executed by parties other than the Company and the
Trust, we have assumed that such parties had the power, corporate or other, to
enter into and perform all obligations thereunder and have also assumed the due
authorization by all requisite action, corporate or other, and execution and
delivery by such parties of such documents and the validity and binding effect
thereof on such parties. As to any facts material to the opinions expressed
herein which we did not independently establish or verify, we have relied upon
oral or written statements and representations of officers, trustees and other
representatives of the Company, the Trust and others.


OnBank Capital Trust I 
ONBANCorp, Inc.
June 23, 1997
Page 3


            Members of our firm are admitted to the Bar of the State of
Delaware, and we do not express any opinion as to the laws of any other
jurisdiction.

            Based upon and subject to the foregoing, we are of the opinion that:

      1. The Capital Securities have been duly authorized for issuance by the
Trust, and when (i) the Registration Statement becomes effective and the
Declaration has been qualified under the Trust Indenture Act of 1939, as
amended, and (ii) the Capital Securities are duly executed, authenticated and
issued in accordance with the Declaration and delivered and issued in the
Exchange Offer as contemplated by the Registration Rights Agreement and the
Registration Statement, the Capital Securities will represent, subject to the
qualifications set forth in paragraph 2 below, fully paid and nonassessable
undivided beneficial interests in the assets of the Trust.

      2. The holders of the Capital Securities will be entitled to the same
limitation of personal liability extended to stockholders of private
corporations for profit organized under the General Corporation Law of the State
of Delaware. We bring to your attention, however, that the holders of the
Capital Securities may be obligated, pursuant to the Declaration, to (i) provide
indemnity and/or security in connection with, and pay taxes or governmental
charges arising from, transfers of Capital Securities and the issuance of
replacement Capital Securities and (ii) provide security and indemnity in
connection with requests of or directions to the Property Trustee and to
exercise its rights and powers under the Declaration.


OnBank Capital Trust I 
ONBANCorp, Inc.
June 23, 1997
Page 4


            We hereby consent to the filing of this opinion with the Commission
as an exhibit to the Registration Statement. We also consent to the reference to
our firm under the caption "Validity of Exchange Securities" in the Registration
Statement. In giving this consent, we do not thereby admit that we are included
in the category of persons whose consent is required under Section 7 of the Act
or the rules and regulations of the Commission. This opinion is expressed as of
the date hereof, and we disclaim any undertaking to advise you of any subsequent
changes in the facts stated or assumed herein or of any subsequent changes in
applicable law.

                                    Very truly yours,

                                    /s/ Skadden, Arps, Slate,
                                      Meagher & Flom LLP