Its surface area of 4.67 km2 (1.80 sq mi) makes it, by this metric, the twentieth largest on the island.

It is located a short distance inland from Sōya Bay, with which it is connected by the lower reaches of the Koetoi River, and is six kilometers east of the urban center of Wakkanai.

It is also known as Shupunto Swamp from its Ainu name meaning "swamp belly" Ōnuma provides a resting point in the autumn for Whooper swans, mute swans, white-fronted geese, and grey heron from Siberia on their way to their winter grounds, and again in spring upon their return.

On October 11, 1943, the Wakkanai detachment's floatplanes attacked the submarine USS Wahoo (SS-238), which was attempting to break through the Sōya Strait, with depth charges and guided it to surface ships, contributing to its sinking.

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