The O Brook is a short right-bank tributary of the West Dart River on Dartmoor in Devon, England.

It lies near the village of Hexworthy, and the lower part of the stream forms the boundary between the parishes of Holne and Dartmoor Forest (formerly Lydford).

[1] The area around the stream is notable for its historic tin mines.

[2] A rock formation on the stream bank, known as the Dragon's Den, gave rise to the legend of the Dragon of the O Brook.

This article related to a river in England is a stub.