• Ocaria Clench, 1970
• Olearia Herrmannsen, 1847 [Family: Turbinidae]
• Olearia Moench, 1802 [Family: Asteraceae]
• Ollaria V.P. Maslov, 1955 †
• Olybria Heinrich, 1956
• Omaria Grube, 1869 accepted as Ophelina Örsted, 1843
• Ontaria Clarke, 1904 (uncertain)
• Oplaria Losana, 1829 (uncertain)
• Oryzaria Agassiz, 1846 (unaccepted) [Class: Protozoa (awaiting allocation)]
• Oryzaria Defrance, 1825 † accepted as Alveolina d'Orbigny, 1826 † [Class: Tubothalamea]
• Oscaria Gray, 1873 [Family: Trionychidae]
• Oscaria Lilja, 1839 (uncertain) [Family: Primulaceae]
• Oscaria Vosmaer, 1881 accepted as Oscarella Vosmaer in Bronn, 1884 [Family: Oscarellidae]
• Oskaria Ospanova, 1983 †
• Oslaria Dyar, 1904
• Osmaria Razowski, 1991
• Otaria Kunth ex G. Don, 1837 (uncertain) [Family: Apocynaceae]
• Otaria Péron in Péron & Lesueur, 1816 [Family: Otariidae]
• Otaria Shaw, 1800 [Family: Otariidae]
• Otaria Zhdanko, 1984 (uncertain) [Family: Lycaenidae]
• Ottaria Hedqvist, 1974 (uncertain)
• Ovaria P.C. Fabricius accepted as Solanum Linnaeus, 1753
• Oxneria S.Y. Kondratyuk & Kärnefelt, 2003
• Oxyacris Willemse, 1957 (uncertain)
• Oxyaeida Bolívar, 1914
• Oxyaena Cope, 1874 †
• Oxyalcia Dognin, 1905
• Oxyaporia Townsend, 1919 (uncertain)
• Oxybia Hampson in Romanoff, 1901
• Oxycara Solier, 1835
• Oxycarmia de Laubenfels, 1954 accepted as Mycale Gray, 1867
• Oxycarpia Trautschold, 1875 † (uncertain)
• Oxydia Guenée, 1857
• Oxygia Walker, 1856 accepted as Yigoga Nye, 1975
• Oxyharma Boucek, 1988
• Oxylamia Breuning, 1944
• Oxylapia Kiener & Maugé, 1966
• Oxylia Mulsant, 1863
• Oxymyia Kertész, 1916
• Oxynaia Haas, 1913
• Oxynia Pascoe, 1885
• Oxyparga Uvarov, 1953
• Oxyparna Korneyev, 1990
• Oxypria Kieffer, 1908 (uncertain)
• Oxyria J. Hill, 1765 [Family: Polygonaceae]
• Oxyria Yu, 2005 accepted as Forcipomyia Meigen, 1818 [Family: Ceratopogonidae]
• Oxytria Rafinesque, 1837 (unaccepted)
• Oxytrita Warren in Seitz, 1913
• Oxyura A.P. de Candolle, 1836 (uncertain) [Family: Asteraceae]
• Oxyura Agassiz, 1846 accepted as Oxura Kirby, 1819 [Family: Tenebrionidae]
• Oxyura Bonaparte, 1831 [Family: Anatidae]
• Oxyurias Stiles in Stiles & Hassall, 1905 accepted as Enterobius Baird, 1853
• Oxyuris Rudolphi, 1803