Kanagawa Prefecture Ōi has a humid subtropical climate (Köppen Cfa) characterized by warm summers and cool winters with light to no snowfall.

On April 1, 1889, the district of Ashigarakami was administratively divided into villages, including Kaminaka, Soga, Yamada and Kaneda with the establishment of the modern municipalities system.

Ōi has a mayor-council form of government with a directly elected mayor and a unicameral town council of 14 members.

Ōi, together with the other municipalities in Ashigarakami District and Minamiashigara city, collectively contributes one member to the Kanagawa Prefectural Assembly.

In terms of national politics, the town is part of Kanagawa 17th district of the lower house of the Diet of Japan.