The Oker is a river in Lower Saxony, Germany. It is a tributary of the Aller river and 105 km in length.

The source is located in the Harz mountains, where the river winds through a narrow valley called the Okertal. It then runs northwards through Vienenburg[?] and Wolfenbüttel before entering Brunswick, the largest city on the banks. Some 30 km north of Brunswick the Oker joins the Aller river.