- On that score
- On the anvil
- On the dot
- On the files
- On the part
- On the scent
- On the sly
- On the spur of the moment
- On the stocks
- On the stroke
- On the table
- On the up and up
- On time
- On top of the world
- On view
- Once bitten twice shy
- Once bitten, twice shy
- Once for all
- Once or twice
- Once-and-for-all
- One among the ten forms of similie
- One another
- One hundred
- One of the four main directions of the compass
- One that can excel
- One who acts
- One who eats
- One who erects
- One who has a share
- One who has attained majority
- One who has not performed the ceremony of tonsure
- One who has taken an additional wife
- One who ignores household duties as laid down by the scriptyres
- One who lives by his writing
- One who mastered the arts
- One who punishes
- One who reprimands
- One with another
- One with deformed limbs of without limbs
- One-act play
- One-handed
- One-horse
- One-way street
- Onerousness
- Ones
- Ones right hand
- Oniony
- Online
- Onliness
- Onset
- Ontological
- Ontologically
- Onus
- Onward
- Onwardness
- Oof
- Oogenesis
- Oology
- Oomph
- Oops-a-daisy
- Ooze
- Oozing out
- Op
- Opal
- Opalescence
- Open and above board
- Open letter
- Open relationship
- Open sesame
- Open space
- Open to doubt
- Open-air theatre
- Open-minded
- Opening door
- Opera glass
- Operate
- Operation
- Operation theatre
- Operative
- Ophite
- Ophthalmic
- Ophthalmology
- Opinable
- Opinion poll
- Opium-den
- Opium-eater
- Opportunity
- Oppose
- Opposite
- Opposite angle
- Opposite side
- Opposite view
- Oppressed
- Oppression
- Oppressive
- Opprobrious
- Oppugnancy
- Optic centre
- Optical scanner
- Optics