Oregon Revised Statutes Crimes and Punishments § 161.055

(1) When a “defense,” other than an “affirmative defense” as defined in subsection (2) of this section, is raised at a trial, the state has the burden of disproving the defense beyond a reasonable doubt.

(2) When a defense, declared to be an “affirmative defense” by chapter 743, Oregon Laws 1971, is raised at a trial, the defendant has the burden of proving the defense by a preponderance of the evidence.

(3) The state is not required to negate a defense as defined in subsection (1) of this section unless it is raised by the defendant. “Raised by the defendant” means either notice in writing to the state before commencement of trial or affirmative evidence by a defense witness in the defendant's case in chief.