It has an area of 425 square kilometers (164 sq mi) [1] The island consists of two stratovolcanos connected by a relatively flat isthmus.

The largest is Ozernaya River (8 kilometers (5.0 mi) long, which flows from the southern part of the caldera of the Nemo volcano to the Sea of Okhotsk.

The climate of Onekotan is characterized by short, cool summers, high humidity, frequent fogs, and especially strong winds.

Onekotan still has an active volcano with the most recent eruption, in 1952, forming a small lava dome on the coast of Krenitsyn island within the lake of Tao-Rusyr Caldera.

Onekotan, with a number of shallow bays and sandy beaching providing landing points, was inhabited by the Ainu prior to European contact.

The strong winds and harsh climate limit the growth of trees to small thickets of Krummholz formations of Siberian dwarf pine with a maximum height of 2–4 meters, mostly in gullies.