The Osse is a 120 km (75 mi) long river in southwestern France, right tributary of the river Gélise. Its source is in the Hautes-Pyrénées, 2 km (1.2 mi) southeast of the village Bernadets-Debat, in the Plateau de Lannemezan. It joins the river Gélise 5 km (3.1 mi) southwest of the town Nérac.