Dodatak:Abecedni popis diapsida: O
Oc
uredi- Occitanosaurus Bardet, Godefroit & Sciau, 1999 †
- Ocoyuntaia arquata,
- Ocoyuntaia,
Og
uredi- Ogmodirus Williston & Moodie, 1913
Ol
uredi- Oligodontosauridae,
- Oligodontosaurus
- Olindalacerta,
Om
uredi- Omoiotyphlops,
- Omphalosauridae Merriam, 1906 †
Op
uredi†Porodica Ophthalmosauridae Appleby 1956
uredi- †Arthropterygius Maxwell 2010
- †Baptanodon Seeley 1874
- †Ichthyosaurus doughtyi Seeley 1869
- †Muiscasaurus Maxwell et al. 2015
- †Nannopterygius Huene 1923
- †Ochevia Arkhangelsky 2001
- †Ophthalmosaurinae Baur 1887
- †Platypterygiinae Arkhangelsky 2001
Diapsida → Ichthyopterygia → Parvipelvia → Neoichthyosauria → Thunnosauria
†Potporodica Ophthalmosaurinae Baur 1887
uredi- †Acamptonectes Fischer et al. 2012
- †Cryopterygius Druckenmiller et al. 2012
- †Ichthyosaurus brunsvicensis Broili 1909
- †Janusaurus Robert et al. 2014
- †Leninia Fischer et al. 2013
- †Mollesaurus Fernandez 1999
- †Ophthalmosaurus Seeley 1874
- †Ophthalmosaurus cantabrigiensis Lydekker 1888
- †Palvennia Druckenmiller et al. 2012
- †Paraophthalmosaurus Arkhangelsky 1997
- †Undorosaurus Efimov 1999
Diapsida → Ichthyopterygia → Neoichthyosauria → Thunnosauria → Ophthalmosauridae
- Ophthalmosaurus Seeley, 1874 †
- Opluridae,
- Oplurus
Or
uredi- Orithyia
- Ornithocephalidae,
- Ornithocheiridae Seeley, 1870 †
- Ornithocheiroidea Seeley, 1901 †
- Ornithocheirus Seeley, 1869 †
Klada Ornithodira Gauthier 1986
uredi- Dinosauromorpha Benton 1985
- †Pterosauromorpha Padian 1997
Diapsida → Archosauromorpha → Archosauriformes → Crurotarsi → Archosauria → Avemetatarsalia
- Ornithosoma Seeley, 1871 †
- Ornithosuchia Huene, 1908 †
- Ornithosuchia,
- Ornithosuchidae Huene, 1908 †
- Ornithosuchus Newton, 1894 †
- Orophosaurus,
- Orovenator,
Os
uredi- Osmolskina,
- Osteolaemus Cope, 1861
Ot
uredi- Ototriton,
Ow
uredi- Oweniasuchus †
Ox
uredi- Oxysdonsaurus †