The River Oise (French pronunciation: ​[waz]) is a right tributary of the River Seine, flowing for 302 kilometres (188 mi) in Belgium and France. Its source is in the Belgian province Hainaut, south of the town Chimay. It crosses the border with France after about 20 kilometres (12 mi). It flows into the Seine in Conflans-Sainte-Honorine, near Paris. Its main tributary is the River Aisne.