The Oker rises at about 910 metres in the Harz National Park in a boggy area on the Bruchberg in the Harz mountains of central Germany. This early section is known as the Große Oker (Great Oker) and it is impounded below Altenau by the Oker Dam. From the dam wall to the former village of Oker, which is today part of Goslar, the Oker is on certain occasions suitable for canoeing. This section, often called the Oker Valley (Okertal), includes the Romkerhall Waterfall. Here the Romke stream drops about in height over a waterfall laid out in 1863 into the Oker. Downstream in the river's fast-flowing waters, the Verlobungsinsel (Betrothal Island) is to be found. Left and right of the Oker in this area are many crags that are popular with climbers.