Kleine Oker, Mountain brook in Clausthal-Zellerfeld and Altenau, Germany.

Kleine Oker is a mountain brook that winds through the Harz landscape between Clausthal-Zellerfeld and Altenau. The water flows clear and fresh over rocks and boulders, forming small cascades as it travels through dense forest and open hillsides.

The Kleine Oker rises in the Lilierkopf swamp at 730 m and crosses the Dammgraben, an artificial ditch built from 1732 to supply water power to Upper Harz mines. About a kilometre below that crossing it feeds the Kleiner Okerteich, a former mining pond used as a forest swimming pool since the 1920s. In Altenau the brook runs in pipes and walls past the town hall and joins the Große Oker at the market square after 460 m.

The Kleine Oker gave its name to the Kleiner Okerteich, the former mining pond it feeds about a kilometre below the Dammgraben crossing; since the 1920s that pond has served Altenau as a Waldschwimmbad, a forest swimming pool. At the pool's northern end the brook disappears under a tennis court and comes back out of a concrete pipe.

The brook rises in the Lilierkopf swamp at 730 m and crosses the Dammgraben in its upper course; about a kilometre below that crossing it feeds the Kleiner Okerteich, a former mining pond used as a forest swimming pool since the 1920s. At the pool's northern end a tennis court covers the stream, which reappears from a concrete pipe. In Altenau it runs mostly in walls and pipes, passing underground beside the town hall to the market square, where it meets the Große Oker at 460 m.

The Kleine Oker drains only 1.7 km², yet its water once fed the Kleiner Okerteich, a mining pond that has served as a forest swimming pool since the 1920s. At the pool's northern end the stream disappears under a tennis court and re-emerges from a concrete pipe.