Osage County is a county in the central part of the U.S. state of Missouri. As of the 2020 census, the population was 13,274. Its county seat is Linn. The county was organized January 29, 1841, and named from the Osage River.

The Jefferson City metropolitan statistical area, as defined by the United States Census Bureau, is an area consisting of four counties – Cole, Callaway, Moniteau, and Osage – in central Missouri anchored by the city of Jefferson City. As of the 2020 census, the MSA had a population of 150,316. The Jefferson City MSA consists of four counties, and borders the Columbia metropolitan area to the north.