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                                                                   EXHIBIT 8.1

                          [BARACK FERRAZZANO KIRSCHBAUM
                       PERLMAN & NAGELBERG LLC LETTERHEAD]


                                  May 20, 2003



The Board of Directors
Old Second Bancorp, Inc.
37 River Street
Aurora, Illinois  60506

     Re:  Tax Opinion Regarding Certain Federal Income Tax Consequences of the
          Purchase, Ownership and Disposition of Cumulative Trust Preferred
          Securities to be issued by Old Second Capital Trust I

Ladies and Gentlemen:

     We have acted as special counsel to Old Second Bancorp, Inc., a Delaware
corporation (the "Company") and Depositor of Old Second Capital Trust I, a
Delaware statutory business trust (the "Trust"), in connection with the
preparation of a Registration Statement on Form S-3 (the "Registration
Statement") filed by the Company and the Trust with the Securities and Exchange
Commission under the Securities Act of 1933, as amended (the "Securities Act"),
relating to (i) the proposed issuance by the Trust of $25,000,000 aggregate
Liquidation Amount of the Trust's Cumulative Trust Preferred Securities due
__________, 2033 (the "Preferred Securities") registered under the Securities
Act; (ii) the Company's Subordinated Debentures to be issued by the Company to
the Trust (the "Debentures") registered under the Securities Act; and (iii) the
Company's guarantee (the "Guarantee") of distributions and payments on
liquidation or redemption of the Preferred Securities, as described in and
pursuant to the Preferred Securities Guarantee Agreement (the "Guarantee
Agreement") to be executed by the Company, as Guarantor, and Wilmington Trust
Company, as Guarantee Trustee. The Preferred Securities are issuable under an
Amended and Restated Trust Agreement (the "Trust Agreement") to be entered into
between the Company, as Depositor; Wilmington Trust Company, a Delaware trust
company, as Delaware Trustee and as Property Trustee; and William B. Skoglund,
an individual; J. Douglas Cheatham, an individual; and Robin Hodgson, an
individual, each an Administrative Trustee. The Debentures are issuable under an
indenture (the "Indenture") to be entered into between the Company and
Wilmington Trust Company, as Trustee. Capitalized terms used herein and not
otherwise defined herein shall have the meanings assigned to them in the Trust
Agreement, the Indenture and the Guarantee Agreement, as applicable.

     In rendering our opinion, we have reviewed various documents relating to
the Preferred Securities, the Trust and the Debentures. In our examination, we
have assumed the genuineness
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The Board of Directors
May 20, 2003
Page 2


of all signatures, the legal capacity of natural persons, 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. We have further assumed
that each of the parties to each of the documents, certificates, letters of
representation or other similar materials relating, directly or indirectly, to
the issuance of the Preferred Securities (the "Transaction Documents") and the
Debentures and the creation of the Trust has and will fully comply with all of
its obligations thereunder without amendment or waiver, and that there are not,
and will not be any, arrangements, understandings or agreements among any of the
parties relating to the issuance of the Preferred Securities or the Debentures
and the creation of the Trust other than those expressly set forth in the
Transaction Documents.

     Based on the foregoing, we are of the opinion that, under existing United
States federal income tax law as of the date hereof, assuming the accuracy of
the aforementioned representations and assumptions on the date hereof as well as
continuing satisfaction of such representations and assumptions, although there
is no definitive authority on the facts presented here (i) the Debentures will
be treated as indebtedness of the Company for United States federal income tax
purposes; (ii) the Trust will be classified for United States federal income tax
purposes as a grantor trust and not as an association taxable as a corporation
and, accordingly, each owner of Preferred Securities will be treated as owning
an undivided interest in the Debentures; and (iii) the summary of the United
States federal income tax consequences to the owners of the Preferred Securities
set forth under the heading "MATERIAL FEDERAL INCOME TAX CONSEQUENCES" in the
Registration Statement is materially correct.

     Our opinion is based on the Internal Revenue Code of 1986, the regulations
promulgated thereunder and other relevant authorities and law, all as in effect
on the date hereof. Consequently, future changes in the Internal Revenue Code of
1986, the regulations promulgated thereunder and other relevant authorities and
law may cause the tax treatment of the Preferred Securities, the Debentures and
the Trust to be materially different from that described above. Our opinion
represents only our legal judgment and, unlike a tax ruling, is binding neither
on the Internal Revenue Service nor a court of law, and has no official status
of any kind. The Internal Revenue Service or a court of law could disagree with
the opinions expressed herein.

     This opinion, as qualified and limited herein, is strictly limited to the
characterization of the Debentures and the Preferred Securities and the
classification of the Trust for United States Federal income tax purposes, and
we express no opinion with respect to any other considerations which may arise
relating to the Preferred Securities, the Debentures or the Trust, any other
taxes or any other matters arising under United States federal, state, local or
foreign law.
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The Board of Directors
May 20, 2003
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     This opinion is solely for the benefit of the above-named addressees, and
any subsequent holder of Preferred Securities, and only relates to the
aforementioned Preferred Securities, the Debentures and the Trust, and may not
be relied upon by any other person in any manner whatsoever. This opinion should
not be quoted in whole or in part or otherwise referred to, or relied upon, nor
be filed with or furnished to any governmental agency or other person or entity,
without the prior written consent of this firm.


                                        Very truly yours,

                                        /s/ Barack Ferrazzano Kirschbaum
                                              Perlman & Nagelberg LLC
                                        -------------------------------------
                                        BARACK FERRAZZANO KIRSCHBAUM
                                            PERLMAN & NAGELBERG LLC