Osage County is a county located in the U.S. state of Kansas. Its county seat is Lyndon, and its most populous city is Osage City. As of the 2020 census, the county population was 15,766. The county was originally organized in 1855 as Weller County, then renamed in 1859 after the Osage tribe.

Osage City is a city in Osage County, Kansas, United States. As of the 2020 census, the population of the city was 2,861.