Oregon Revised Statutes Evidence and Witnesses § 40.360

(1) The credibility of a witness may be attacked by evidence that the witness engaged in conduct or made statements showing bias or interest. In examining a witness concerning a prior statement made by the witness, whether written or not, the statement need not be shown nor its contents disclosed to the witness at that time, but on request the statement shall be shown or disclosed to the opposing party.

(2) If a witness fully admits the facts claimed to show the bias or interest of the witness, additional evidence of that bias or interest shall not be admitted. If the witness denies or does not fully admit the facts claimed to show bias or interest, the party attacking the credibility of the witness may then offer evidence to prove those facts.

(3) Evidence to support or rehabilitate a witness whose credibility has been attacked by evidence of bias or interest shall be limited to evidence showing a lack of bias or interest.