- 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
- Opus
- Or what ever
- Or-else
- Oracle
- Oracular
- Orally
- Orange plant
- Oratorial powers
- Orbicular
- Orbit
- Order from the top
- Order of the day
- Ordinary fly
- Ordinary man
- Ordinary run of men
- Orexis
- Organelle
- Organic laws
- Organism
- Orientation
- Origami
- Ornamental
- Ornamental dot
- Ornamentation
- Ornamented
- Orotund
- Orphanize
- Orpiment