http://www.eregulations.com/oregon/fishing/definitions/ Resident: A person who has resided in Oregon at least six months immediately prior to the date of making application for a license, tag or permit issued by the State Fish and Wildlife Commission. Temporary absence from the state for a purpose other than establishing residency outside the state shall not be considered in determining whether a person meets the residency requirement. These requirements are waived for certain active members of the uniformed services and for some foreign exchange students.
as I recall, it isn't a matter of proving how long you have been a resident but stating under penalty of law that you have in fact established permanent residency at least 6 months prior to buying your license/tags.