Ōi (大井町, Ōi-machi) was a town located in Iruma District, Saitama Prefecture, Japan.

As of 2003, the town had an estimated population of 47,276 and a population density of 6,014.76 persons per km. The total area was 7.86 km.

On October 1, 2005, Ōi, along with the city of Kamifukuoka, was merged to create the city of Fujimino.

35°51′04″N 139°31′01″E / 35.851°N 139.517°E / 35.851; 139.517