2011 ORS § 161.249¹
Use of physical force by private person assisting an arrest

(1) Except as provided in subsection (2) of this section, a person who has been directed by a peace officer to assist the peace officer to make an arrest or to prevent an escape from custody is justified in using physical force when and to the extent that the person reasonably believes that force to be necessary to carry out the peace officers direction.

(2) A person who has been directed to assist a peace officer under circumstances specified in subsection (1) of this section may use deadly physical force to make an arrest or to prevent an escape only when:

(a) The person reasonably believes that force to be necessary for self-defense or to defend a third person from what the person reasonably believes to be the use or imminent use of deadly physical force; or

(b) The person is directed or authorized by the peace officer to use deadly physical force unless the person knows that the peace officer is not authorized to use deadly physical force under the circumstances. [1971 c.743 §30]