- Owner-driver
- Obstructing
- Obtund
- Ocean-going
- Octosyllabic
- Off-colour
- Off-color
- Off-day
- Offered
- Office-bearer
- Officer-in-charge
- Off-key
- Off-season
- Off-stage
- Oil-cloth
- Old-fashioned
- Oldish
- Old-hat
- Old-time
- Old-timer
- Old-world
- Olive-oil
- Omnifarious
- On leave
- One-horse
- One-armed
- Oneiric
- One-way
- On-the-spot
- Oozed
- Open-air
- Open-and-shut
- Open-door policy
- Open-handed
- Open-mouthed
- Opera-house
- Orang-utan
- Ordaining
- Occipital
- Optometry
- Oubliette
- Ossiferous
- Ostiole
- Overfeed
- Overproud
- Overshot
- Omnicient