Lake Ohau is a glacial lake in the Mackenzie Basin of New Zealand's South Island. It is fed by the Hopkins and Dobson rivers, which have their headwaters in the Southern Alps, and has its outflow in the Ohau River, which itself feeds into the Waitaki River hydroelectric project.

Ohau is the smallest of three roughly parallel lakes running north-south along the northern edge of the Mackenzie Basin (the others are Lake Pukaki and Lake Tekapo). It covers 60 km².

The Ohau ski field is located close to the southwestern shore of the lake.

The lake forms part of the traditional boundary between Otago and Canterbury regions, Otago's northernmost point being the headwaters of the Hopkins River. Officially the lake lies in the northwestern part of the Waitaki District within the southern part of the Canterbury Region.