It slopes 25 degrees on the west side. The eastern slope is cut by a sea cliff. This cliff was a fault scarp that formed while the cone was growing. The base of Oshima is made of old stratovolcanoes of olivine basalt . The main cone is a stratovolcano made of basalt lava and pyroclastic rocks.
Steep walls surround the caldera except in the northeast.
This part of the wall is buried under lava flows from Mihara-yama. Layers of lavas and pyroclastic materials in the caldera have been faulted and folded. It has an outer slope of about 20 degrees. Its summit crater is filled with lava from a 1950-51 eruption. A small cinder cone was formed by this eruption on the southern rim of the crater.