- order Ornithischia
- order Crocodylia
- order Anguilliformes
- order Struthioniformes
- order Coccidia
- order Insessores
- order Pelecaniformes
- order Passeriformes
- order Temnospondyli
- order Notostraca
- order Coleoptera
- order Apodiformes
- order Chiroptera
- order Corrodentia
- Ornithomimida
- order Stegocephalia
- order Percomorphi
- order Rheiformes
- order Anacanthini
- order Trichoptera
- order Mycrosporidia
- order Geophilomorpha
- order Haplosporidia
- order Foraminifera
- order Perciformes
- order Artiodactyla
- order Siphonophora
- order Scandentia
- order Mecoptera
- order Dinoflagellata
- order Conodontophorida
- order Mallophaga
- order Chelonia
- order Chelonethida
- order Pediculati
- order Trogoniformes
- order Scleroparei
- order Diptera
- order Phasmatodea
- osteochondrosis
- osteodystrophia fibrosa
- orthodontics
- organs of equilibrium
- Oort cloud
- old world coot
- ornicology
- ornithine decarboxylase test
- overall broadcast reliability
- overall reception reliability
- overall response