Body with a central disc of up to 10 mm in diameter and 5 arms of about three times that length; coloured bluish-grey or dark brownish, often with white spots.
The scales of the disc are uniform and rather coarse. Only the central plate is distinct, the other primary plates are usually indistinguishable. The radial shields are small and usually white. The arm combs are only feebly developed, usually consisting only of a small irregular group of papillae on each side at the base of the arm (O. robusta disc).
The dorsal arm plates are rather narrow, with the outer edge being strongly convex. The ventral arm plates are usually widely separated, with a slight concavity in the outer edge and a very obtuse proximal angle. There are 3 arm spines per segment, the uppermost being considerably larger than the other two. Only one small tentacle scale per tube foot pore (O. robusta arm).
The mouth shields are short and broad, scarcely reaching half-way to the edge of the disc. There is a single, vertical row of teeth placed on top of the jaw, and 3-4 small, simple mouth papillae on each side of the jaw (O. robusta mouth).
It is found on a variety of substrata, from about 6-450 m.
In the North Sea this species is found from the Shetland Islands down to Durham on the British east coast. Elsewhere it is distributed northwards Spitzbergen and Greenland, and the Atlantic coasts of North America down to Cape Cod.