Osen is a municipality in Trøndelag county, Norway. It is part of the Fosen region. The administrative centre of the municipality is the village of Steinsdalen. The 387-square-kilometre (149 sq mi) municipality is the 240th largest by area out of the 356 municipalities in Norway. Osen is the 340th most populous municipality in Norway with a population of 904. The municipality's population density is 2.4 inhabitants per square kilometre (6.2/sq mi) and its population has decreased by 2.4% over the previous 10-year period.