In the 2021 congressional session, Oregon passed multiple laws, including SB 493 which sets the prevailing wage as the union-negotiated wage for that area. It used to be determined using an annual employer survey.
See here for more information: https://nwlaborpress.org/2021/06/how-working-people-and-unions-fared-in-the-2021-session-of-the-oregon-legislature/