- occupationology
- occupied
- occupied (with)
- ocimum
- oxycephaly
- ockham's razor
- octal digit
- octangular
- octans
- octave
- octavo
- octavian
- octet
- octette
- october
- october 12
- october 24
- october revolution
- octoberfest
- octogenarian
- octogenerian
- octonary
- octopod
- octopoda
- octopodidae
- octopus
- octothorpe
- octroi
- octroi collector
- oxybutyric acid
- oxygen (gas)
- oxydendrum
- oxydendrum arboreum
- oxygen acid
- oxygen debt
- oxygen deficit
- oxygen mask
- oxygen tent
- oxygenate
- oxygenise
- oxygenize
- oyster cracker
- oxyhaemoglobin
- oxylebius pictus
- oxyhemoglobin
- oxylebius
- oxymoron
- oxyphenbutazone
- oxyphencyclimine
- oxytetracycline