- 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
- Optimal
- Optimism
- Optimize
- Oral sex
- Orange
- Orange tree
- Orate
- Oration
- Orbital
- Orchard
- Orchestrate
- Orchid
- Ordain
- Order out
- Ordinal
- Ordinarily
- Ordinate
- Ordination
- Organ of sense
- Organic
- Organic chemistry
- Organic matter
- Organically
- Organiser
- Organization
- Orgasm
- Orgies
- Oriental
- Orifice
- Originality
- Originated
- Ornamentally
- Ornaments
- Optophone