| Name | Created | Sq.Miles | Population | Cities |
|---|---|---|---|---|
| Baker | 22 Sep 1862 | 3,068 | 16,287 | Baker City, Huntington |
| Benton | 23 Dec 1847 | 676 | 78,640 | Corvallis, Philomath |
| Champoeg | 5 Jul 1843 | became Marion County in 1849 | ||
| Clackamas | 5 Jul 1843 | 1,868 | 368,470 | Lake Oswego, Oregon City |
| Clatsop | 22 Jun 1844 | 827 | 36,798 | Astoria, Seaside |
| Columbia | 16 Jan 1854 | 656 | 48,065 | St. Helens, Scappoose |
| Coos | 22 Dec 1853 | 1,600 | 64,711 | Coos Bay, North Bend |
| Crook | 24 Oct 1882 | 2,979 | 22,067 | Prineville |
| Curry | 18 Dec 1855 | 1,627 | 22,427 | Brookings, Harbor |
| Deschutes | 13 Dec 1916 | 3,018 | 141,382 | Bend, Redmond |
| Douglas | 7 Jan 1852 | 5,036 | 104,202 | Roseburg, Sutherlin |
| Gilliam | 25 Feb 1885 | 1,204 | 1,794 | Condon, Arlington |
| Grant | 14 Oct 1864 | 4,528 | 7,297 | John Day, Prairie City |
| Harney | 25 Feb 1889 | 10,134 | 6,898 | Burns, Hines |
| Hood River | 23 Jun 1908 | 522 | 21,284 | Hood River, Odell |
| Jackson | 12 Jan 1852 | 2,785 | 195,322 | Medford, Ashland |
| Jefferson | 12 Dec 1914 | 1,780 | 20,100 | Madras, Warm Springs |
| Josephine | 22 Jan 1856 | 1,639 | 80,761 | Grants Pass, Redwood |
| Klamath | 17 Oct 1882 | 5,944 | 66,192 | Altamont, Klamath Falls |
| Lake | 24 Oct 1874 | 8,135 | 7,313 | Lakeview, Paisley |
| Lane | 28 Jan 1851 | 4,554 | 335,180 | Eugene, Springfield |
| Lincoln | 20 Feb 1893 | 979 | 45,994 | Newport, Lincoln City |
| Linn | 28 Dec 1847 | 2,292 | 108,914 | Albany, Lebanon |
| Malheur | 17 Feb 1887 | 9,887 | 31,330 | Ontario, Nyssa |
| Marion | 1849 | 1,183 | 305,265 | Salem, Keizer |
| Morrow | 16 Feb 1885 | 2,032 | 11,666 | Boardman, Irrigon |
| Multnomah | 22 Dec 1854 | 435 | 672,906 | Portland, Gresham |
| Polk | 22 Dec 1845 | 741 | 70,295 | Dallas, Monmouth |
| Sherman | 25 Feb 1889 | 823 | 1,749 | Wasco, Moro |
| Tillamook | 15 Dec 1853 | 1,102 | 25,277 | Tillamook, Rockaway Beach |
| Tuality | 1843 | became Washington County in 1849 | ||
| Umatilla | 27 Sep 1862 | 3,215 | 73,878 | Pendleton, Hermiston |
| Umpqua | 1851 | merged with Douglas County in 1862 | ||
| Union | 14 Oct 1864 | 2,036 | 24,540 | La Grande, Union |
| Wallowa | 11 Feb 1877 | 3,145 | 7,014 | Enterprise, Joseph |
| Wasco | 11 Jan 1854 | 2,381 | 23,593 | City of The Dalles, Chenoweth |
| Washington | 3 Sep 1849 | 723 | 499,794 | Beaverton, Hillsboro |
| Wheeler | 17 Feb 1899 | 1,714 | 1,455 | Fossil, Mitchell |
| Yamhill | 5 Jul 1843 | 715 | 92,196 | McMinnville, Newberg |
| Total | 95,996 | 3,641,056 | ||
Source: U. S. Census Bureau; using estimates of 2005 population.
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