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. Osage County is part of the Jefferson City, MO Metropolitan Statistical Area. Its geography and the founding of Westphalia Vineyards links it to the Missouri Rhineland, extending along the Missouri River valley to the western edges of the St. Louis Metropolitan Area.