Oregon Revised Statutes Evidence and Witnesses § 44.552

(2)(a) A person causing a subpoena to be issued to compel the attendance of a police officer or an employee of the Department of State Police before a tribunal shall indicate on the face of the subpoena whether the person or the person's representative intends to ask the expert opinion of the officer or employee as to any aspect of the proceedings.

(b) A police officer or an employee of the Department of State Police may not be required by a tribunal to give the officer's or employee's expert opinion on any matter before the tribunal unless the subpoena compelling the officer's or employee's presence indicates that the officer's or employee's expert opinion will be asked.