- ostracoderm
- ostracodermi
- ostrava
- ostrea
- ostrea gigas
- ostreidae
- ostrich
- ostrich fern
- ostrogoth
- ostrya
- ostrya carpinifolia
- ostrya virginiana
- ostryopsis
- ostwald
- ostwald's theory of indicators
- ostyak
- ostyak-samoyed
- oswald
- oswald spengler
- oswald veblen
- oswego tea
- 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
- o'toole
- 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
- ouachita
- ouachita river
- oubliette
- ouguiya
- ouija
- ouija board
- oujda
- ounce
- our lady's bedstraw
- our lady's mild thistle
- our lord's candle
- ouranopithecus
- ouranos
- ouse
- ouse river
- ousel
- oust
- ouster
- ousting
- out
- out and away
- out front
- out in
- out loud
- out of
- out of bounds
- out of doors
- out of gear
- out of hand
- out of nothing
- out of place
- out of play
- out of practice
- out of print
- out of reach
- out of sight
- out of stock
- out of the blue
- out of the question
- out of the way
- out of thin air
- out of true
- out of use
- out of view
- out of wedlock
- out of whack
- out of work
- out or keeping
- out to
- outage
- out-and-out
- out-and-outer
- outaouais
- outback
- outbalance
- out-basket
- outbid
- outboard
- outboard motor
- outboard motorboat
- outbound
- outbrave
- outbreak
- outbred
- outbuilding
- outburst
- outcall
- outcast
- outcaste
- outclass
- outclassed
- outcome
- outcrop
- outcropping
- outcry
- outdated
- outdistance
- outdo
- outdoor
- outdoor game
- outdoor man
- outdoor sport
- outdoor stage
- outdoors
- outdoorsman
- outdoorswoman
- outdoorsy
- outdraw
- outer
- outer boundary
- outer ear
- outer garment
- outer hebrides
- outer mongolia
- outer planet
- outer space
- outercourse