The Oust (Breton: Oud) is a river in Brittany, France, right tributary of the Vilaine. Its source is in the hills between Corlay and Quintin. It flows generally southeast, through the following départements and towns:The Oust flows into the river Vilaine in Redon. The part of the Oust between Rohan and Redon has been made navigable for small ships, and forms part of the Canal de Nantes à Brest.The river Arz is among its tributaries.