O-shima (ō-shē´mä), island, c.35 sq mi (90 sq km), near the entrance to Tokyo Bay, E Japan. The largest and most northerly of the Izu-shichito group, it is the site of volcanic Mt. Mihara (2,477 ft/7,550 m), which last erupted in 1986. Agriculture and fishing are chief activities there. It is a recreation area for Tokyo. The island was visited (17th cent.) by Maarten Vries, the Dutch navigator.