COUNTY FORMATION
| NAME | FORMED | PARENT COUNTY | COUNTY SEAT |
| BAKER | 1862 | Wasco | Baker City |
| BENTON | 1847 | Polk | Corvallis |
| CHAMPOEG | 1843 | Renamed Marion | in 1849 |
| CLACKAMAS | 1843 | [Original] | Oregon City |
| CLATSOP | 1844 | Tuality | Astoria |
| COLUMBIA | 1854 | Washington | St. Helens |
| COOS | 1853 | Jackson/Umpqua | Coquille |
| CROOK | 1882 | Wasco | Prineville |
| CURRY | 1855 | Coos | Gold Beach |
| DESCHUTES | 1916 | Crook | Bend |
| DOUGLAS | 1852 | Umpqua | Roseburg |
| GILLIAM | 1885 | Wasco/Morrow | Condon |
| GRANT | 1864 | Umatilla/Wasco | Canyon City |
| HARNEY | 1889 | Grant | Burns |
| HOOD RIVER | 1908 | Wasco | Hood River |
| JACKSON | 1852 | Umpqua | Medford |
| JEFFERSON | 1914 | Crook | Madras |
| JOSEPHINE | 1856 | Jackson | Grants Pass |
| KLAMATH | 1882 | Lake | Klamath Falls |
| LAKE | 1874 | Jackson/Wasco | Lakeview |
| LANE | 1851 | Linn/Umpqua/Benton | Eugene |
| LINCOLN | 1893 | Benton/Polk | Newport |
| LINN | 1847 | Champoeg | Albany |
| MALHEUR | 1887 | Baker | Vale |
| MARION | 1843 [Original] | as Champoeg | Salem |
| MORROW | 1885 | Umatilla | Heppner |
| MULTNOMAH | 1854 | Clackamas/Washington | Portland |
| POLK | 1845 | Yamhill | Dallas |
| SHERMAN | 1889 | Wasco | Moro |
| TILLAMOOK | 1853 | Clatsop/Yamhill/Polk | Tillamook |
| TUALITY | 1843 | Renamed Washington | in 1849 |
| UMATILLA | 1862 | Wasco | Pendleton |
| UMPQUA | 1851 | Became part of | Douglas in 1862 |
| UNION | 1864 | Baker | La Grande |
| WALLOWA | 1887 | Union | Enterprise |
| WASCO | 1854 | Marion/Clackamas/Linn | The Dalles |
| WASHINGTON | 1843 [Original] | as Tuality | Hillsboro |
| WHEELER | 1899 | Crook/Gilliam/Grant | Fossil |
| YAMHILL | 1843 | [Original] | McMinnville |