- On the whole
- On the wrong track
- On thin ice
- Onanism
- Once
- Once bitten is twice shy
- Once in a life time
- Once more and again
- Oncology
- One after another
- One and the same
- One man
- One million
- One place
- One practised in adulation
- One roaming
- One short of
- One too many
- One which deserves to be imitated
- One who chants sama veda
- One who dedicates
- One who is inactive
- One who pities
- One who shaves
- One who submits
- One who throws
- One-man show
- One-mindedness
- One-size-fits-all
- One-up
- Oneirology
- Oneration
- Orientalize
- Ones meat
- Ones mouth
- Oneself
- Oniomania
- Online service
- Only resort
- Only-begotten
- Onomasticon
- Onomatoepic
- Onrush
- Onslaught
- Onto
- Onymous
- Oodles
- Oof bird
- Oological
- Oophorectomy
- Oosperm
- Ootheca
- Ooze from
- Oozing latex
- Opalescent
- Opalize
- Opaque
- Open fire
- Open mind
- Open newel
- Operatic
- Open question
- Open secret
- Open the floodgates
- Open the pandoras box
- Open university
- Open up
- Open-air
- Opening
- Openness
- Opera-house
- Operational guidelines
- Operator
- Operculum
- Operetta
- Opex
- Opine
- Opium
- Opossum
- Opponent
- Opportuneness
- Opposite bank
- Opposite course
- Oppositely
- Oppositive
- Oppressiveness
- Oppressor
- Oppugnent
- Opt
- Opt out
- Optative
- Optic circle
- Optical
- Orange peel
- Optimistic
- Optimum
- Option
- Optionally
- Optometrist
- Optometry