- Otaheite arrowroot
- Otaheite arrowroot starch
- otalgia
- Otaria
- Otaria Byronia
- Otariidae
- otc
- OTC market
- OTC security
- OTC stock
- Othello
- other
- other than
- otherness
- otherwise
- otherworld
- otherworldliness
- otherworldly
- Othman I
- Otho
- Otho I
- Otho of Lagery
- othonna
- otic
- otic ganglion
- Otides
- Otididae
- otiose
- Otis
- Otis Skinner
- Otis tarda
- otitis
- otitis externa
- otitis interna
- otitis media
- Oto
- Otoe
- otoganglion
- otolaryngologist
- otolaryngology
- otologist
- otology
- OToole
- otoplasty
- otorhinolaryngologist
- otorhinolaryngology
- otorrhea
- otosclerosis
- otoscope
- ototoxic
- ottar
- ottava rima
- Ottawa
- Ottawa river
- otter
- otter hound
- otter shrew
- otterhound
- Ottmar Mergenthaler
- Otto Frisch
- Otto Fritz Meyerhof
- Otto Hahn
- Otto Heinrich Warburg
- Otto I
- Otto Jespersen
- Otto Loewi
- Otto Meyerhof
- Otto Neumann Sverdrup
- Otto Robert Frisch
- Otto the Great
- Otto von Bismarck
- Otto Wagner
- Ottoman
- Ottoman dynasty
- Ottoman Empire
- Ottoman Turk
- Ottorino Respighi
- Ottumwa
- Otus
- Otus asio
- Otus scops
- Otus sunia