The municipal unit Oinountas covers the area between the northeastern part of the Evrotas valley up to the ridge of Parnon Mountain. The river Oinountas flows through the southeastern part of the municipal unit.
The municipality Oinountas was first established in 1835, the seat of administration being Vamvakou, then Vresthena in 1840. It was abolished in 1912, and the municipality was split into the independent communities of Vamvakou, Vresthena, Vasaras and Arachova. It was then re-founded by law 2539/1997 (Kapodistrias Plan) in 1998, covering a larger area, by the merger of the communities Sellasia (the seat of the new municipality), Vasaras, Theologos, Varvitsa, Vamvakou, Koniditsa, Voutianoi and Vresthena.