Ōyano-jima (大矢野島), also spelt as Ōyanojima It is administered as part of the city of Kami-Amakusa.

Large part of island belongs to Unzen-Amakusa National Park.

Ōyano-jima is an irregular shape, with a ragged, elongated outline oriented at north-south axis.

The highest mount is Tobidake (飛岳) which stands at 229 m. Ōyano-jima is the third largest island in the Amakusa group lying west of Kyushu, Japan.

Precipitation is significant throughout the year; The summer tends to be Ōyano-jima's wettest season, with the tsuyu (梅雨 tsuyu, "plum rain") — the rainy season — occurring between early June (average:Jun.7) to late July (average:Jul.21).