Mother and grandmother both were residents and domiciled in CA at time of death. As far as I can tell, the probate starts in CA with ancillary proceedings in OR.

Prob C § 7051:
"If the decedent was domiciled in this state at the time of death, the proper county for proceedings concerning administration of the decedent’s estate is the county in which the decedent was domiciled, regardless of where the decedent died."