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 Indian murders were a series of murders of Osage in Osage County, Oklahoma, during the 1910s–1930s. Newspapers described the increasing number of unsolved murders and deaths among young adults as the "Reign of Terror". Most took place from 1921 to 1926. Some sixty or more wealthy, full-blood Osage persons were reported killed from 1918 to 1931. Newer investigations indicate that other suspicious deaths during this time could have been misreported or covered-up murders, including those of individuals who were heirs to future fortunes. Further research has shown that the death toll may have been in the hundreds.