Body long, cylindrical, flattened anteriorly.
Prostomium triangular, sligthly longer than broad; without eyes.
Thorax with 22-24 chaetigers. Thoracic notopodia with a single postchaetal lobe, an acicula and a bundle of capillary chaetae. Thoracic neuropodia are tori with smooth or denticulate spines and capillaries. In the first five chaetigers, thoracic neuropodia with semicircular postchaetal lamella, later with 10-15 conical postchaetal papillae until chaetiger 27-31. Subpodial papillae on the ventral side of neuropodia 17-20 to 27-35. No spines.
Abdominal notopodia with simple postchaetal lobes, aciculae, capillary and forked chaetae. Cirriform interramal cirrus about as long as the neuropodium. Abdominal neuropodia bilobed, with one or two aciculae, a few capillary chaetae and a small ventral cirrus.
Gills long, lanceolate and ciliated, starting at chaetiger 5 (O. sertulata-parapodium).
Pygidium with two long cirri.
Up to 300 mm for 400 segments.
Thorax dark red, abdomen greyish-brown.
On sand or mud, upper to lower sublittoral.
Arctic, Northeast Atlantic to Mediterranean, North Sea to Bay of Kiel.