- Otus asio
- orthostatic hypotesion
- ottumwa
- odontoma
- orson welles
- oreortyx picta palmeri
- or to deceive
- opisthobranchia
- otides
- onion stem
- or a discipline
- oregon fir
- origanum dictamnus
- otho i
- oregon jargon
- Oriental arborvitae
- Oregon larch
- Oregon cedar
- order coniferales
- Omphalotus
- Ozonium
- order Phallales
- Oligoporus
- Ophiodontidae
- Osmeridae
- Ogcocephalidae
- Osteoglossidae
- Ophidiidae
- Orectolobidae
- Opisthognathidae
- Orthopristis
- Orectolobus
- Oligoplites
- Ophiodon
- Osmerus
- Oriental garlic
- one who has a liking for exquisiteness and excellence
- overneutralisation
- Oto
- Otoe
- Ohm's law
- operational damage
- operational casualty
- overcompensation
- one who leads astray
- oryx
- on_the_scene
- odonate
- ortega y gasset
- Ossete