Oseto, Nagasaki

 was a town located in Nishisonogi District, Nagasaki Prefecture, Japan.
As of 2003, the town had an estimated population of 7,577 and a density of 96.25 persons per km². The total area was 78.72 km².
On April 1, 2005, Ōseto, along with the towns of Saikai (former), Ōshima, Sakito and Seihi (all from Nishisonogi District), was merged to create the city of Saikai.