Ōseto, Nagasaki

Ōseto (大瀬戸町) 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.