The Oker (pronounced [ˈoːkɐ]) is a river in Lower Saxony, Germany, that has historically formed an important political boundary.

After these two confluences the river continues southeast past the Harly Forest, after which it bends north to flow through Schladen and Wolfenbüttel to Braunschweig.

In the Bürgerpark shortly before Braunschweig's old town the Oker divides into the western and eastern bypass channels (Umflutgraben) which circumnavigate the historic city centre at a slightly higher level.

Following the merger of the two channels northwest of the city centre the Oker runs north of the district of Watenbüttel [de] in a culvert under the Mittelland Canal before it is joined by the Schunter from the east near Groß Schwülper.

Since the early ninth century the middle Oker river has formed the diocesan boundary between the bishoprics of Halberstadt and Hildesheim, established by Emperor Charlemagne and his son Louis the Pious in the Duchy of Saxony.

The Bishopric of Halberstadt was likewise mediatised in 1803, and according to the Final Act of the 1815 Vienna Congress, the Oker was the eastern border of the Kingdom of Hanover with the Duchy of Brunswick and the Prussian Province of Saxony.