Ōiso is a town located in Kanagawa Prefecture, Japan. As of 1 January 2023, the town had an estimated population of 31,262 and a population density of 1820 persons per km². The total area of the town is 17.18 square kilometres (6.63 sq mi).

Kanagawa Prefecture is a prefecture of Japan located in the Kantō region of Honshu. Kanagawa Prefecture is the second-most populous prefecture of Japan at 9,221,129 and third-densest at 3,800 inhabitants per square kilometre (9,800/sq mi). Its geographic area of 2,415 km2 (932 sq mi) makes it fifth-smallest. Kanagawa Prefecture borders Tokyo to the north, Yamanashi Prefecture to the northwest and Shizuoka Prefecture to the west.