The first county seat, Thomasville, was laid out on Eleven Point River in 1845 on a site given by John and Matilda Thomas. In 1859, the county seat was moved by law to a more central location and Alton, a new town, was founded. In the Civil War, Union troops burned the courthouse there in 1863. Through the war the county, largely pro-Southern, suffered guerrilla raids and troop movement. A post war outlaw band routed by the county militia in 1868.
Thayer, the largest town in the county, was founded as railway division point, 1881, on the newly built St. Louis, Ft. Scott & Memphis (Frisco) R.R. Early stations were Koshkonong, St. Elmo, and American.
Noted for its splendid scenery, Oregon is a lumbering and livestock farming county. In an arc, through the county, runs lovely, spring-fed Eleven Point River. Hundreds of prehistoric Indian mounds were found along the river and its tributaries.
In northeastern Oregon and in adjoining counties lies the historic Irish Wilderness where in 1858, Father John Hogan founded a Catholic colony. By 1859, forty families, many of them Irish, settled the area. A colony chapel was built s.e. [sic] of Wilderness village in Oregon County near the Ripley County line. Civil War activities ended the colony adventure, and the area is now part of Clark National Forest, founded in the 1930s1
Near Koshkonong is famed Grand Gulf, an extensive chasm made by a collapsed cave. The county's large, beautiful springs, Blue, Boze, Turner, Falling, Thomasson, and Greer, were all pioneer mill sites. At Greer, third largest spring in the Ozarks, power was sent by cable to the mill on the hilltop from a turbine at the spring in the valley. The second largest spring in the Ozarks, Mammoth, is south of Thayer, in Arkansas.