Osage County (county code OS) is a county located in the U.S. state of Kansas. As of the 2010 census, the county population was 16,295. Its county seat is Lyndon, and its most populous city is Osage City. The county was originally organized in 1855 as Weller County, and was renamed in 1859 after the Osage River that runs through it, which is itself named for the Osage Native American Tribe.