1. Obeliscoides Graybill 1924
2. Obliquogobius Koumans 1941
3. Oceania Péron & Lesueur 1810
4. Ochetellus Shattuck 1992
5. Ochotona Link 1795
6. Ochridacyclops Kiefer 1937
7. Octolasmis Gray 1825
8. Octomeris Sowerby 1825
9. Octophialucium Kramp 1955
10. Ocynectes D.S. Jordan & Starks 1904
11. Ocypode Weber 1795
12. Ocyropsis Mayer 1912
13. Odacantha Paykull 1798
14. Odobenus Brisson 1762
15. Odontepyris Kieffer 1904
16. Odontesthes Evermann & Kendall 1906
17. Odontobutis Bleeker 1874
18. Odontomachus Latreille 1804
19. Odontoponera Mayr 1862
20. Oecophylla Smith 1860
21. Oedomyzon Stock 1981
22. Oenone Savigny 1818
23. Oesophagodontus Bailliet & Henry 1902
24. Oesophagostomum Molin 1861
25. Ogmocotyle Skrjabin & Schuhz 1933
26. Ohbayashinema Durette-Desset 1974
27. Ohomopterus Reitter 1896
28. Oiceoptoma Leach 1815
29. Oithona Baird 1843
30. Okadaia de Laubenfels 1936
31. Okamejei Ishiyama 1958
32. Okapinema Ivashkin 1960
33. Okinawasega Terayama 1999
34. Oldroydia Dall 1894
35. Oligoaeschna Sélys 1889
36. Oligolepis Bleeker 1874
37. Oligometra A.H. Clark 1908
38. Oligomyrmex Mayr 1867
39. Oligozaptyx Pilsbry 1905
40. Oliva Bruguière 1789
41. Ollulanus Leuckart 1865
42. Omaseus Dejean 1821
43. Omobranchus Ehrenberg 1836
44. Omosita Erichson 1843
45. Omphalius Philippi 1847
46. Onchocerca Diesing 1841
47. Onchocleidus Mueller 1936
48. Oncomelania Gredler 1881
49. Oncorhynchus Suckley 1861
50. Ondatra Link 1795
51. Onigocia Jordan & Thompson 1913
52. Onithochiton Gray 1847
53. Onithoplax Thiele 1910
54. Onuxodon Smith 1955
55. Onychocamptus Daday 1903
56. Onychodactylus Tschudi 1838
57. Ophelina Örsted 1843
58. Ophiactis Lütken 1856
59. Ophiarachna Müller & Troschel 1842
60. Ophichthus Ahl 1789
61. Ophidiaster L. Agassiz 1836
62. Ophioblennius Gill 1860
63. Ophiocoma Agassiz 1835
64. Ophiodromus Sars 1861
65. Ophiomaza Lyman 1871
66. Ophiophthalmus Matsumoto 1917
67. Ophioplocus Lyman 1861
68. Ophiothela Verrill 1867
69. Ophisurus Lacepède 1800
70. Ophiura Lamarck 1816
71. Ophrynopus Konow 1897
72. Opisthoplatia Brunner von Wattenwyl 1865
73. Opisthosyllis Langerhans 1879
74. Opistognathus G. Cuvier 1816
75. Oplegnathus Richardson 1840
76. Oplophorus H. Milne Edwards 1837
77. Opsariichthys Bleeker 1863
78. Orectolobus Bonaparte 1834
79. Oreochromis Günther 1889
80. Orientabia Malaise 1934
81. Orientelmis Shepard 1998
82. Orientobdella Epshtein 1962
83. Orientostrongylus Durette-Desset 1971
84. Orionella JedliĊka 1963
85. Ormyrus Westwood 1832
86. Ornithodiplostomum Dubois 1936
87. Ornithoptera Boisduval 1832
88. Ornithostrongylus Travassos 1914
89. Orthetrum Newman 1833
90. Orussus Latreille 1797
91. Oryza Linné
92. Oryzias D.S. Jordan & Snyder 1906
93. Oslerus Hall 1921
94. Osphronemus Lacépède 1801
95. Ostertagia Ransom 1907
96. Ostorhinchus Lacepède 1802
97. Ostracion Linnaeus 1758
98. Ostrea Linnaeus 1758
99. Ostreobdella Oka 1927
100. Ostrincola Wilson C.B. 1944
101. Oswaldocruzia Travassos 1917
102. Otostrongylus de Bruyn 1933
103. Otus Pennant 1769
104. Oulangia Milne-Edwards & Haime 1848
105. Oulastrea Milne-Edwards & Haime 1848
106. Ovachlamys Habe 1946
107. Oxalis Linné
108. Oxycentrus Chaudoir 1854
109. Oxycheilinus Gill 1863
110. Oxycomanthus Rowe, Hoggett, Birtles & Vail 1986
111. Oxypora Saville-Kent 1871
112. Oxyurichthys Bleeker 1874
113. Oxyuris Rudolphi 1803
114. Ozobranchus De Quatrefages 1852