Oregon Revised Statutes Crimes and Punishments § 161.245

(1) For the purposes of ORS 161.235 and 161.239, a reasonable belief that a person has committed an offense means a reasonable belief in facts or circumstances which if true would in law constitute an offense. If the believed facts or circumstances would not in law constitute an offense, an erroneous though not unreasonable belief that the law is otherwise does not render justifiable the use of force to make an arrest or to prevent an escape from custody.

(2) A peace officer who is making an arrest is justified in using the physical force prescribed in ORS 161.235 and 161.239 unless the arrest is unlawful and is known by the officer to be unlawful.