O. robusta has a hook-shaped middle spine on distal arm segments. This character is unusual in Ophiura and my suggest that the species belongs in a different genus, possibly Ophiuroglypha.
Thuy, B.; Stöhr, S. (2011). Lateral arm plate morphology in brittle stars (Echinodermata: Ophiuroidea): new perspectives for ophiuroid micropalaeontology and classification. Zootaxa. 3013, 1-47.