Saint-Ost is a commune in the Gers department in southwestern France.

The Petite Baïse forms part of the commune's southeastern border, flows north through the middle of the commune, then forms part of the commune's northeastern border.

Historical population

Year	Pop.	±%
1962	139	—
1968	137	−1.4%
1975	108	−21.2%
1982	105	−2.8%
1990	94	−10.5%
1999	86	−8.5%
2008	81	−5.8%

• Communes of the Gers department