From Ancestry.com Wiki
This entry was originally written by Dwight A. Radford for Red Book: American State, County, and Town Sources.
Foreign-born residents could become naturalized in any court of record. Records created prior to 1906 may still be in the county. Some records were not kept in separate naturalization books, but in court journals or court minutes, making them difficult to locate. The Oregon State Archives has a large collection of citizenship records.