Ōsa (大佐町 Ōsa-chō) was a town located in Atetsu District, Okayama, Japan.
On March 31, 2005 Ōsa, along with the towns of Shingō, Tessei and Tetta, all from Atetsu District, was merged into the expanded city of Niimi.
As of 2003, the town had an estimated population of 3,922 and a density of 32.35 persons per km². The total area was 121.25 km².