ORS § 41.740
When the terms of an agreement have been reduced to writing by the parties, it is to be considered as containing all those terms, and therefore there can be, between the parties and their representatives or successors in interest, no evidence of the terms of the agreement, other than the contents of the writing, except where a mistake or imperfection of the writing is put in issue by the pleadings or where the validity of the agreement is the fact in dispute. However this section does not exclude other evidence of the circumstances under which the agreement was made, or to which it relates, as defined in ORS 42.220, or to explain an ambiguity, intrinsic or extrinsic, or to establish illegality or fraud. The term "agreement" includes deeds and wills as well as contracts between parties.
ORS 41.740