Ōseto (大瀬戸町 Ōseto-chō) was a town located in Nishisonogi District, Nagasaki, 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 was merged with the towns of Saikai (old), Ōshima, Sakito and Seihi, all from Nishisonogi District, to form the new city of Saikai.