Osage County (county code OS) is a county in the U.S. state of Kansas. In 2010, 16,295 people lived there. Its county seat is Lyndon. 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.