Oregon Revised Statutes Public Officers and Employees § 241.265

(1) The appointing power shall appoint to each vacant position one of the candidates certified. The person so appointed shall be on probation for a period to be fixed by the regulations of the commission, not to exceed one year if the position is in the police department of the office of the sheriff, otherwise not to exceed six months. During probation the appointing power may discharge the probationer, and in like manner appoint another of such candidates, and so continue until all candidates have been appointed. The reasons, in each instance, for such discharge, shall be filed in writing with the commission. If the reasons are deemed insufficient by the commission, the discharged candidate shall, notwithstanding such discharge, retain the rank upon the eligible list.

(2) When any person who has taken a promotional examination, and who has been appointed to a position in a higher classification or grade on probation, fails to qualify for the position in the higher classification or grade within the probationary period, the person shall not lose seniority in the lower classification or grade from which such promotion was made but shall return to employment and be reinstated in the position held by the person in such lower classification or grade.