The Ōhau River is a river in the Mackenzie Basin of New Zealand's South Island. It is the primary outflow of Lake Ōhau, from which it flows in a roughly eastward direction until reaching the artificial Lake Benmore after a distance of roughly 27 kilometres (17 mi). In 1981, the artificial Lake Ruataniwha was created along the river's length to power a small hydroelectric power station as part of the Waitaki hydro scheme. Prior to the creation of the Waitaki hydro scheme, the river flowed directly into the Waitaki River rather than through a series of lakes. The river forms part of the traditional boundary between Otago and Canterbury regions.