Oregon Naturalization and Citizenship
In the territorial era, immigrants could apply for citizenship at any U.S. district court. Naturalization records filed as part of the “donation land” laws are at the National Archives.
After statehood in 1859, the circuit court had primary jurisdiction over naturalization. The county clerk served as clerk of the circuit court and kept the records. You can obtain copies of declarations and petitions from the clerk's office in each county. Some naturalization records may also be found in county court journals or U.S. district court records. The Family History Library has copies of some naturalization records for Oregon.
For naturalization records after September 1906, contact the National Archives—Pacific Northwest Region (Seattle) or the local office of the United States Citizenship and Immigration Services (formerly INS). The Seattle Branch has records of the U.S. District Court for 1859 to 1970, and the U.S. Circuit Court for 1870 to 1912. The Family History Library has U.S. District Court indexes 1863-1956 and records 1859-1941 (beginning on FHL film 1492135 item 2), and U.S. Circuit Court indexes 1859-1956 (beginning on FHL film 1433979). There is also a published index covering the years 1906-1930, W. David Samuelsen, Oregon Naturalization Records Index (FHL book 979.5 P4s). The library also has military petitions covering 1917-1918.
Information about Oregon Naturalization can be found on the internet. Use a search engine and key works such as Oregon Naturalization and Citizenship.