Ōiso is located on the coastline of central Kanagawa Prefecture, facing Sagami Bay of the Pacific Ocean.

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

[2] Per Japanese census data,[3] the population of Ōiso has grown steadily over the past century, with a slight drop from 2010 to 2020.

Blessed with a temperate climate, and with convenient access to Tokyo due to the Tōkaidō Main Line railway, it was favoured as a seaside health resort by politicians and literary figures during the Meiji period after a glowing report on its location was written by noted physician Matsumoto Jun.

This popularity continued into the postwar era, and Prime Minister Shigeru Yoshida also had a residence in Ōiso to which he retired after leaving politics.

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