Os is a municipality in Innlandet county, Norway. It is located in the traditional district of Østerdalen. The administrative centre of the municipality is the village of Os i Østerdalen. The municipality is located to the west of the municipality of Røros and to the south of the municipalities of Midtre Gauldal and Holtålen in Trøndelag county. In Innlandet county, Os is located to the west of the municipality of Tolga and to the north of the municipality of Engerdal.

The 1,040-square-kilometre (400 sq mi) municipality is the 110th largest by area out of the 356 municipalities in Norway. Os is the 288th most populous municipality in Norway with a population of 1,855. The municipality's population density is 1.8 inhabitants per square kilometre (4.7/sq mi) and its population has decreased by 9.1% over the previous 10-year period.