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                                                                       EXHIBIT 8
 
                                February 9, 1998
 
First Empire State Corporation
One M&T Plaza
Buffalo, New York 14240
 
ONBANCorp, Inc.
101 South Salina Street
P.O. Box 4983
Syracuse, New York 13221-4983
 
Ladies and Gentlemen:
 
    Reference is made to the information set forth under the heading "PROPOSED
MERGER--Certain Federal Income Tax Consequences" contained in the Joint Proxy
Statement-Prospectus, which is included in the Registration Statement on Form
S-4 (the "Registration Statement"), filed by First Empire State Corporation
("First Empire") with the Securities and Exchange Commission (the "SEC") in
connection with the solicitation of proxies by the Boards of Directors of First
Empire and ONBANCorp, Inc. ("ONBANCorp") for their use at their respective
special meetings of stockholders, at which stockholders of First Empire and
ONBANCorp will be asked to approve and adopt an Agreement and Plan of
Reorganization and related Agreement and Plan of Merger, each dated as of
October 28, 1997. Subject to the representations, assumptions and other
conditions described or referenced in this letter and under that heading, it is
our opinion that the discussion of anticipated material federal income tax
consequences contained under that heading is accurate in all material respects.
 
    Our opinion is based on the case law, Internal Revenue Code, Treasury
Regulations and Internal Revenue Service rulings as they exist at the date
hereof. These authorities are all subject to change, and any such change may be
made with retroactive effect. We can give no assurance that, after such change,
our opinion would not be different. We undertake no responsibility to update or
supplement our opinion following the effective date of the Registration
Statement.
 
    We hereby consent to the filing with the SEC of this opinion as an exhibit
to the Registration Statement and to the reference to our firm under the heading
"PROPOSED MERGER--Certain Federal Income Tax Consequences" contained therein. In
giving such consent, we do not thereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act of 1933.
 
                                          Very truly yours,
 
                                           /s/ Arnold & Porter
                 ---------------------------------------------------------------
 
                                          ARNOLD & PORTER
