Oregon County is a county located in the southern portion of the U.S. state of Missouri. As of the 2020 census, the population was 8,635. Its county seat is Alton. The county was officially organized on February 14, 1845, and was named for the Oregon Territory in the northwestern United States. Home to a large area of the Mark Twain National Forest, Oregon County contains more national forest acreage than any county in the state of Missouri.