The Osse () is a long river in southwestern France, right tributary of the river Gélise. Its source is in the Hautes-Pyrénées, southeast of the village Bernadets-Debat, in the Plateau de Lannemezan. It joins the river Gélise southwest of the town Nérac.
Its course crosses the following départements and communes:
Hautes-Pyrénées: Bernadets-Debat Gers: Montesquiou, Saint-Arailles, Vic-Fezensac, Mouchan Lot-et-Garonne: Fréchou, Moncrabeau, Lannes
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