Bill O'Reilly today quoted from a criminal court decision regarding
the fact that a criminal defense attorney does not have the obligation
to seek out the truth and is allowed to try to confuse or otherwise
negate the testimony of a witness, etc. Can you please help me find
the citation?  Thank you

Hello.

Bill O'Reilly was probably quoting late Justice Byron White's
dissenting opinion in the U.S. Supreme Court case United States v.
Wade, 388 U.S. 218 (1967).

In the opinion which is available on the web site of Findlaw.com,
Justice White wrote:

"Law enforcement officers have the obligation to convict the guilty
and to make sure they do not convict the innocent. They must be
dedicated to making the criminal trial a procedure for the
ascertainment of the true facts surrounding the commission of the
crime. 5 To this extent, our so-called adversary system is not
adversary at all; nor should it be. But defense counsel has no
comparable obligation to ascertain or present the truth. Our system
assigns him a different mission. He must be and is interested in
preventing the conviction of the innocent, but, absent a voluntary
plea of guilty, we also insist that he defend his client whether he is
innocent or guilty. The State has the obligation to present the
evidence. Defense counsel need present nothing, even if he knows what
the truth is. He need not furnish any witnesses to the police, or
reveal any confidences of his client, or furnish any other information
to help the prosecution's case. If he can confuse a witness, even a
truthful one, or make him appear at a disadvantage, unsure or
indecisive, that will be his normal course. 6 Our interest in not
convicting the innocent permits counsel to put the State to its proof,
to put the State's case in the worst possible light, regardless of
what he thinks or knows to be the truth. Undoubtedly there are some
limits which defense counsel must observe 7 but more often than not,
defense counsel will cross-examine a prosecution witness, and impeach
him if he can, even if he thinks the witness is telling the truth,
just as he will attempt to destroy a witness who he thinks is lying.
In this respect, as part of our modified adversary system and as part
of the duty imposed on the most honorable defense counsel, we
countenance or require conduct which in many instances has little, if
any, relation to the search for truth."
http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&court=us&vol=388&page=218

I located the citation to Justice White's dissent in Robert Bennett's
article "Ethics, Zealous Advocacy, and the Criminal Defense Attorney"
on the web site of Cardozo.net:
http://www.cardozo.net/life/winter2001/bennett/

search strategy: "defense counsel," "search for truth"

I hope this helps.

This researcher found exactly what I was looking for almost
immediately after I submitted the question. Great Job & Thank You!