Oregon Revised Statutes Crimes and Punishments § 161.235

(1) To make an arrest or to prevent the escape from custody of an arrested person unless the peace officer knows that the arrest is unlawful; or

(2) For self-defense or to defend a third person from what the peace officer reasonably believes to be the use or imminent use of physical force while making or attempting to make an arrest or while preventing or attempting to prevent an escape.