1. Oceaniidae Eschscholtz 1829
2. Ochotonidae Thomas 1897
3. Oculinidae Gray 1847
4. Ocypodidae Rafinesque 1815
5. Ocyropsidae Harbison & Madin 1982
6. Odobenidae Allen 1880
7. Odontaspididae J.P. Müller & Henle 1839
8. Odontobutidae Hoese & A.C. Gill 1993
9. Oenonidae Kinberg 1865
10. Ogcocephalidae Gill 1893
11. Oithonidae Dana 1853
12. Okadaiidae Baba 1930
13. Oleaceae Hoffmannsegg & Link 1809
14. Olindiidae Haeckel 1879
15. Olividae Latreille 1825
16. Ollulanidae Skryabin & Shikhobalova 1952
17. Ommastrephidae Steenstrup 1857
18. Onchidiidae Rafinesque 1815
19. Onchocercidae Chabaud & Anderson 1959
20. Opecoelidae Ozaki 1925
21. Opheliidae Malmgren 1867
22. Ophiacanthidae Ljungman 1867
23. Ophiactidae Matsumoto 1915
24. Ophichthidae Günther 1870
25. Ophidiasteridae Verrill 1870
26. Ophidiidae Rafinesque 1810
27. Ophiocomidae Ljungman 1867
28. Ophiodermatidae Ljungman 1867
29. Ophiolepididae Ljungman 1867
30. Ophiotrichidae Ljungman 1867
31. Ophiuridae Müller & Troschel 1840
32. Opistognathidae Bonaparte 1835
33. Oplegnathidae J. Richardson 1840
34. Oplophoridae Dana 1852
35. Oreasteridae Fisher 1911
36. Orectolobidae Gill 1896
37. Ormyridae Förster 1856
38. Orussidae Newman 1834
39. Osmeridae D.E. McAllister 1963
40. Osphronemidae Bleeker 1859
41. Ostraciidae Rafinesque 1810
42. Ostreidae Rafinesque 1815
43. Otariidae Gray 1825
44. Oxalidaceae R. Brown 1818
45. Oxyuridae Weinland 1858
46. Ozobranchidae Pinto 1921