1. Posterior half of carapace behind spinous cervical groove smooth............................................... 2
– Posterior half of carapace behind spinous cervical groove with scattered spinules or with post-cervical groove bearing cincture of spines or spinules organized in transverse rows........................................................... 3
2. Cervical groove cincture of spines with fewer than 26 spines. Transverse grooves on first three abdominal somites. Dorsotransverse carina on sixth abdominal somite, posterior pleural teeth...................... O. edwardsii (Bouvier, 1908)
– Cervical groove cincture of spines with more than 26 spines. Abdomen smooth.................... O. lopheliae sp. nov.
3. Posterior half of carapace behind spinous cervical groove with scattered and inconspicuous spines elsewhere not arranged in transverse rows........................................................ O. anaphorae Manning & Chace, 1990
– Posterior half of carapace behind spinous cervical groove with post-cervical groove bearing cincture of spines or spinules organized in transverse rows................................................................................ 4
4. Propodus of third pereiopod with dorsal margin without spines, with 2 rows of long setae on dorsal and ventral margins................................................................................ O. meloi Anker & Tavares, 2013
– Propodus of third pereiopod with no rows of long setae on dorsal and ventral margins.............................. 5
5. Pleuron of abdominal somite 3 with 3 medial teeth........................................... O. striata Goy, 1981
– Pleuron of abdominal somite 3 with 0–2 medial teeth........................................................ 6
6. Pleura of abdominal somites 3 and 4 without medial teeth..................................... O. libertae Gore, 1981
– Pleura of abdominal somites 3 and 4 with 1 or 2 medial teeth................................................... 7
7. Pleura of abdominal somite 3 and 4 with one medial tooth....................... O. minoica Dounas & Kokouras, 1989
– Pleura of abdominal somites 3 and 4 with two medial teeth................................... O. addaia Pretus, 1990