- oil of vitriol
- ottoman
- overweary
- oviduct
- old maid
- offense
- opprobrious
- operator
- orientation
- odium
- onanism
- onion
- outpouring
- opossum
- oil gland
- overcome
- ordinary
- opine
- Oread
- orchestrate
- otitis
- offering
- oldness
- oscitancy
- oculist
- odd
- oddly
- odontology
- odor
- odorous
- of a sort
- of age
- of course
- of late
- oxeye
- oxford
- oxford movement
- oxonian
- outweigh
- ouzo
- ovary
- over against
- overacting
- overactive
- overarch
- overbalance
- overbid
- overburden
- overcrop
- overdo