Oker Confluence of the Oker and Aller near Müden Country Germany Physical characteristics Main source Harz Mountains 900 metres (3,000 ft) 51°46′42″N 10°29′29″E﻿ / ﻿51.77833°N 10.49139°E﻿ / 51.77833; 10.49139 River mouth Aller 52°31′26″N 10°21′39″E﻿ / ﻿52.52389°N 10.36083°E﻿ / 52.52389; 10.36083Coordinates: 52°31′26″N 10°21′39″E﻿ / ﻿52.52389°N 10.36083°E﻿ / 52.52389; 10.36083 Length 128.2 km (79.7 mi) [1] Basin features Progression Aller→ Weser→ North Sea Basin size 1,822 km2 (703 sq mi) [1]