Os is a former municipality in the old Hordaland county, Norway. It was located in the Midhordland region, just south of Norway's second-largest city, Bergen. Due to its proximity to Bergen, Os experienced strong population growth. The administrative centre of Os was the village of Osøyro. It is the largest settlement in the municipality, with over 60% of the municipal residents living here. Other large villages in Os included Hagavik, Halhjem, Søfteland, Søre Øyane, and Søvik. On 1 January 2020, the municipality became part of Bjørnafjorden Municipality in Vestland county.

Hordaland was a county in Norway, bordering Sogn og Fjordane, Buskerud, Telemark, and Rogaland counties. Hordaland was the third largest county, after Akershus and Oslo, by population. The county government was the Hordaland County Municipality, which is located in Bergen. Before 1972, the city of Bergen was its own separate county, apart from Hordaland. On 1 January 2020, the county was merged with neighbouring Sogn og Fjordane county, to form the new Vestland county.