Oceanodroma leucorhoa (Vieillot, 1818)
Ochotona pusilla (Pallas, 1769)
Octopus macropus Risso, 1826 – synoniem of Callistoctopus macropus (Risso, 1826)
Odobenus rosmarus (Linnaeus, 1758)
Odocoileus virginianus (Zimmermann, 1780)
Odontochile sp. Hawle & Corda, 1847
Oiceoptoma thoracicum (Linnaeus, 1758)
Okapia johnstoni (P. L. Sclater, 1901)
Oncicola spirula (Olfers in Rudolphi, 1819)
Ondatra zibethicus (Linnaeus, 1766)
Oophaga lehmanni (Myers & Daly, 1976)
Ophiophagus hannah (Cantor, 1836)
Oreaster reticulatus (Linnaeus, 1758)
Oriolus oriolus (Linnaeus, 1758)
Ornithorhynchus anatinus (Shaw, 1799)
Orthetrum cancellatum (Linnaeus, 1758)
Orthotomus sepium Horsfield, 1821
Orycteropus afer (Pallas, 1766)
Oryctolagus cuniculus (Linnaeus, 1758)
Osmoderma eremita (Scopoli, 1763)
Otocyon megalotis (Desmarest, 1822)
Otolemur crassicaudatus (Geoffroy Saint-Hilaire, 1812)
Otolithes ruber (Bloch & Schneider, 1801)
Otolithus argenteus Cuvier, 1830 – synoniem of Otolithes ruber (Bloch & Schneider, 1801)