The island lies only 200 meters north of Natsudomari Peninsula in Mutsu Bay.

It was once possible to walk from the island to the peninsula at low-tide, but erosion made this impossible over time.

The grassland was used for pastoral farming before the island was designated a natural park.

Ōshima is a sill that was formed underground when a volcano dominated the landscape of Natsudomari Peninsula during the Miocene.

[4] The island was designated as a part of Asamushi-Natsudomari Prefectural Natural Park in June 1953.