thumb | right | Bonji Main Hall (Kumagaya City, Saitama Prefecture)
was a town located in Ōsato District, Saitama Prefecture, Japan.
As of 2003, the town had an estimated population of 8,325 and a density of 534.34 persons per km<sup>2</sup>. The total area was 15.58 km<sup>2</sup>.
On October 1, 2005, Ōsato, along with the town of Menuma (also from Ōsato District), was merged into the expanded city of Kumagaya and no longer exists as an independent municipality.