- Overlie
- Overly
- Overmuch
- Overplay
- Overpower
- Overproduce
- Overreach
- Override, overrule
- Overrun
- Oversee
- Overset
- Overshoot oneself
- Overstatement
- Overstep
- Overtime
- Overtone
- Overtones
- Overview
- Overwhelm
- Ovine
- Oviposit
- Oviposition
- Ovule
- Owe person a grudge
- Owe to
- Owls
- Own brand
- Oxails
- Oxbridge
- Oxidation
- Oxide
- Oxidize
- Oxyacetylene
- Octal
- Orchestrics
- Orientate
- Osteoarthritis
- Osteopath
- Olio
- Oak ling
- Oasis
- Oat
- Oatmeal
- Obdurately
- Obediently
- Obeisant
- Obesity
- Obituary
- Obscene language
- Objectify
- Objectiveness
- Objectivity
- Objector
- Obligate
- Obligatory
- Oblige
- Obliged
- Obliging
- Oblong pond
- Obloquy
- Obscenity
- Obsequies
- Obsequiously
- Observant
- Observing
- Obsessed
- Obstetric
- Obstetrician
- Obstruct
- Obstructed
- Obstructions
- Obtainable
- Obtained
- Obtrusive
- Occasion
- Occidental
- Occlude
- Occurred at dawn
- Occurrence
- Ocean-going
- Oceania
- Ochlophobia
- Ochre
- Octagon
- Octane
- Octave
- Octavo
- October
- Octuple
- Odd job man
- Oddly
- Odor
- Odourless
- Oedema
- Oedipus
- Oem
- Of hunters
- Of ones own making
- Of the lip
- Optophone