Osage County is the largest county by area in the U.S. state of Oklahoma. Created in 1907 when Oklahoma was admitted as a state, the county is named for and is home to the federally recognized Osage Nation. The county is coextensive with the Osage Nation Reservation, established by treaty in the 19th century when the Osage relocated there from Kansas. The county seat is in Pawhuska, one of the first three towns established in the county. The total population of the county as of 2020 was 45,818.

The Osage Nation is a Midwestern American tribe of the Great Plains. The tribe developed in the Ohio and Mississippi river valleys around 700 B.C. along with other groups of its language family. They migrated west after the 17th century, settling near the confluence of the Missouri and Mississippi rivers, as a result of Iroquois expansion into the Ohio Country in the aftermath of the Beaver Wars.