The Oise is a river of Belgium and France, flowing for 341 km from its source in the Belgian province of Hainaut, south of Chimay. It crosses the border with France after about 20 km. It flows into the Seine at Conflans-Sainte-Honorine, a north-western suburb of Paris. Its main tributary is the Aisne. It gave its name to the French departments of Oise and Val-d'Oise.