The Oder is a 56-kilometre-long (35 mi) river in Lower Saxony, Germany, and a right tributary of the Rhume.

Its source is located in the southern part of the Brockenfeld about 1.3 km (0.81 mi) north of the Achtermannshöhe.

After the Oder passed under the B 4 federal highway in Oderbrück [de] in east-west direction, it is impounded for the first time west of the village in the Oderteich, where it bends to the south.

In Bad Lauterberg the Oder swings west, running through Pöhlde and Hattorf am Harz, passing north of the Rotenberg ridge, and reaches Wulften.

In Hattorf it collects the Sieber river coming from the northeast, before emptying into the Rhume (a tributary of the Leine) in Katlenburg from the east.