Osen is a municipality in Sør-Trøndelag county, Norway. It is part of the Fosen region. The administrative centre of the municipality is the village of Steinsdalen. Osen is one of two Norwegian municipalities that do not have any non-western immigrants as of 1 January 2008.
The municipality of Osen was established on 1 June 1892 when the old municipality of Bjørnør was split into three municipalities. The initial population of Osen was 1,575. The municipal boundaries have not changed since.The municipality (originally the parish) is named after the old Osen farm (Old Norse: Óss), since the first church was built there. The name is identical with the word óss which means "mouth of a river" (here Steinselva). Osen ("the Os") is a later finite form of óss.