Oediceros parvimanus Bate and Westwood, 1862 – synonym of Westwoodilla caecula
Onisci scolopendroidis Pallas, 1772 – synonym of Caprella linearis
Oniscoda maculosa Latreille, 1829 – synonym of Janira maculosa
Oniscus aquaticus Linnaeus, 1758 – synonym of Asellus aquaticus
Oniscus arenarius Slabber, 1769 – synonym of Haustorius arenarius
Oniscus Balticus Pallas, 1772 – synonym of Idotea baltica
Oniscus chelipes Pallas, 1766 – synonym of Idotea chelipes
Oniscus cicada Fabricius, 1780 – synonym of Tmetonyx cicada
Oniscus entomon Linnaeus, 1758 – synonym of Saduria entomon
Oniscus gammarellus Pallas, 1766 – synonym of Orchestia gammarellus
Oniscus marinus Fabricius, 1780 – synonym of Jaera albifrons
Oniscus psora Linnaeus, 1758 – synonym of Aega psora
Oniscus viridis Slabber, 1775 – synonym of Idotea chelipes
Oniscus volutator Pallas, 1766 – synonym of Corophium volutator
Opis eschrichtii Kröyer, 1842 – synonym of Opisa eschrichtii
Opis quadrimana Bate and Westwood, 1868 – synonym of Normanion quadrimanus
Opis typica Kröyer, 1846 – synonym of Opisa eschrichtii
Orchestia brevidigitata Bate and Westwood, 1868 – synonym of Orchestia gammarellus
Orchestia deshayesii Audouin, 1826 – synonym of Talorchestia deshayesii
Orchestia gryphus Müller, 1848 – synonym of Talorchestia deshayesii
Orchestia hottae Stebbing, 1906 – synonym of Orchestia cavimana
Orchestia laevis Bate, 1857d – synonym of Orchestia mediterranea
Orchestia littorea Leach, 1814 – synonym of Orchestia gammarellus
Orchestia remyi roffensis – synonym of Orchestia roffoensis
Orchestia roffensis – synonym of Orchestia roffoensis
Orchestia trigonocheirus Bate, 1862 – synonym of Orchestia mediterranea
Orchestoidea brito Auct. – synonym of Pseudorchestoidea brito
Orchomene batei Sars, 1882 – synonym of Orchomene humilis
Orchomene goesii Boeck, 1871 – synonym of Nannonyx goesi
Orchomene hanseni Meinert, 1890 – synonym of Orchomene humilis
Orchomenella ciliata Sars, 1891 – synonym of Orchomene nanus
Orchomenella nana Chevreux and Fage, 1925 – synonym of Orchomene nanus
Orchomenella nanus Stebbing, 1906 – synonym of Orchomene nanus
Orchomenella pinguis Sars, 1895 – synonym of Orchomene pinguis
Otus carinatus Bate, 1862 – synonym of Odius carinatus