| Name
| Created
| County Seat
| Sq.Miles
| 2010 Pop.
|
|---|
| Baker
| 22 Sep 1862 | Baker City | 3,068 | 16,134 |
| Benton
| 23 Dec 1847 | Corvallis | 676 | 85,579 |
| Champoeg
| 5 Jul 1843 | became Marion County in 1849 |
| Clackamas
| 5 Jul 1843 | Oregon City | 1,868 | 375,992 |
| Clatsop | 22 Jun 1844 | Astoria | 827 | 37,039 |
| Columbia
| 16 Jan 1854 | St. Helens | 656 | 49,351 |
| Coos
| 22 Dec 1853 | Coquille | 1,600 | 63,043 |
| Crook
| 24 Oct 1882 | Prineville | 2,979 | 20,978 |
| Curry
| 18 Dec 1855 | Gold Beach | 1,627 | 22,364 |
| Deschutes
| 13 Dec 1916 | Bend | 3,018 | 157,733 |
| Douglas
| 7 Jan 1852 | Roseburg | 5,036 | 107,667 |
| Gilliam
| 25 Feb 1885 | Condon | 1,204 | 1,871 |
| Grant
| 14 Oct 1864 | Canyon City | 4,528 | 7,445 |
| Harney
| 25 Feb 1889 | Burns | 10,134 | 7,422 |
| Hood River
| 23 Jun 1908 | Hood River | 522 | 22,346 |
| Jackson
| 12 Jan 1852 | Medford | 2,785 | 203,206 |
| Jefferson
| 12 Dec 1914 | Madras | 1,780 | 21,720 |
| Josephine
| 22 Jan 1856 | Grants Pass | 1,639 | 82,713 |
| Klamath
| 17 Oct 1882 | Klamath Falls | 5,944 | 66,380 |
| Lake
| 24 Oct 1874 | Lakeview | 8,135 | 7,895 |
| Lane
| 28 Jan 1851 | Eugene | 4,554 | 351,715 |
| Lincoln
| 20 Feb 1893 | Newport | 979 | 46,034 |
| Linn
| 28 Dec 1847 | Albany | 2,292 | 116,672 |
| Malheur
| 17 Feb 1887 | Vale | 9,887 | 31,313 |
| Marion
| 1849 | Salem | 1,183 | 315,335 |
| Morrow
| 16 Feb 1885 | Heppner | 2,032 | 11,173 |
| Multnomah
| 22 Dec 1854 | Portland | 435 | 735,334 |
| Polk
| 22 Dec 1845 | Dallas | 741 | 75,403 |
| Sherman
| 25 Feb 1889 | Moro | 823 | 1,765 |
| Tillamook
| 15 Dec 1853 | Tillamook | 1,102 | 25,250 |
| Tuality
| 1843 | became Washington County in 1849 |
| Umatilla
| 27 Sep 1862 | Pendleton | 3,215 | 75,889 |
| Umpqua
| 1851 | merged with Douglas County in 1862 |
| Union
| 14 Oct 1864 | La Grande | 2,036 | 25,748 |
| Wallowa
| 11 Feb 1877 | Enterprise | 3,145 | 7,008 |
| Wasco
| 11 Jan 1854 | The Dalles | 2,381 | 25,213 |
| Washington
| 3 Sep 1849 | Hillsboro | 723 | 529,710 |
| Wheeler
| 17 Feb 1899 | Fossil | 1,714 | 1,441 |
| Yamhill
| 5 Jul 1843 | McMinnville | 715 | 99,193 |
| 36 counties as of 1916 | Total
| 95,996 | 3,831,074 |