The Oster (; , Ostyor) is a river in the northern Ukrainian oblast of Chernihiv. The river is a left tributary of the Desna River. It is approximately 199 km long and its basin area is 2,950 km<sup>2</sup>. It is connected by canals and streams with the Trubizh River, which flows southwest from Kyiv into the Dnieper River. Important towns and villages on the river include: Nizhyn, Kozelets and Oster.