High Quality Content by WIKIPEDIA articles! Osage County (county code OS) is a county located in east-central Kansas, in the Central United States. As of the 2010 census, the county population was 16295. Its county seat is Lyndon, and its most populous city is Osage City. The county along with Shawnee, Jackson, Jefferson, and Wabaunsee counties is included in the Topeka, Kansas, Metropolitan Statistical Area. Osage was originally organized in 1855 as Weller County, and was renamed in 1859 after the Osage River that runs through it.