Oregon Revised Statutes Evidence and Witnesses § 40.040

When part of an act, declaration, conversation or writing is given in evidence by one party, the whole on the same subject, where otherwise admissible, may at that time be inquired into by the other; when a letter is read, the answer may at that time be given; and when a detached act, declaration, conversation or writing is given in evidence, any other act, declaration, conversation or writing which is necessary to make it understood may at that time also be given in evidence.