Oberaarsee is a hydroelectric reservoir in the Grimsel area, part of the municipality of Guttannen, Switzerland. Its surface area is 1.47 km2 (0.57 sq mi). The Oberaar dam was completed in 1953, and is operated by Kraftwerke Oberhasli. It is drained by the Oberaarbach, which flows into the Grimselsee.
The Oberaar glacier is located west of the lake.