Oshima as an island stratovolcano with a large caldera on its summit. Stratovolcanoes are conical volcanoes built up by many layers of hardened lava and volcanic ash, with steep profiles and periodic eruptions.
The main stratovolcano cone of the island has a base composed of basalt lava and pyroclastic rocks. When an erupting volcano empties a shallow magma chamber, the volcanic formation can collapse into the voided reservoir and produce a cauldron-like depression.
Based on geological evidence, it is estimated that Oshima has erupted over 100 times since 8450 B.C. As of 2014, the last eruption was in 1990 at the summit of Mihara-yama.