This delicate species with an oval body may reaches a length of up to 22 mm and a width of 13 mm. The colour is whitish or yellowish, especially along the margins, or brown blending at margins into a band of bluish-grey; the mid-dorsal line and the lateral branches are brownish or carmine. The marginal tentacles are well developed. Each of the tentacles is speckled with about 50 eyes; the cerebral eye clusters contain about 20-30 eyes each and they may be confluent anteriorly. The mouth is situated anteriorly to or beneath the cerebral organ. The intestinal trunk opens posteriorly through a median pore and possesses three or four pairs of lateral branches, which ramify without anastomosing (O. sanguinolentus-diagrammatic). The hinder end of the intestinal trunk ends in a dorsal anus.
The male and female copulatory organs are ventral to the pharynx. The uterine canals separate posteriorly, each with one to four uterine vesicles.
Under stones in the intertidal zone, among red algae and Laminaria holdfasts, dredged on shells and sandy mud in depths to 60 m.
Ranges from Scandinavia to western Mediterranean. Reported also from Wales and Firth of Clyde, Channel Islands, Plymouth Sound, Isle of Man and various Irish localities.