This is one of the most common dorids in the intertidal and also one of the largest, reaching 40 mm in length, pale in ground colour but with blotchy brown markings on the dorsum (rare individuals, usually juveniles, may be pale all over). The ample mantle bears abundant spiculose club-like tubercles of various sizes, usually lacking the brown pigment. There may be as many as 29 separate simple pinnate gills with, characteristically, a few tubercles within the branchial circlet. The pH of the mantle may drop abruptly to 1-3 on abrupt disturbance as defensive acid secretions are released. Each rhinophore may bear up to sixteen lamellae. The head is dilated to form an oral veil without tentacular processes. The spawn is a short flat ribbon laid in a curve or concertina shape (O. fusca-eggs).
This species feeds upon a variety of acorn barnacles; examination of dead prey showed that the cirri and other remains of the chitinous exoskeleton had been left behind, the dorids having taken mainly the soft parts. It seems likely that the juveniles may attack encrusting bryozoans such as Cryptosula and Umbonula .
Common in the North Sea (Distr. O. fusca). Elsewhere it is known from Greenland and Iceland to the Atlantic coast of France, and from both coasts of north America, to 250 m.