- 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
- Of up
- Off ones feet
- Off and on
- Off kilter
- Off limits
- Off stage
- Off the peg
- Off the rails
- Off the top of ones head
- Off-handedly
- Off-road
- Off-time
- Off-wind
- Offence
- Offender
- Offending
- Offered
- Offering
- Offerings of flowers
- Office
- Office bearer
- Office-seeker
- Official
- Official despatches
- Officiate
- Officinal
- Officious
- Offish ness
- Oft
- Oftenness
- Ogle
- Ogress
- Oikos
- Oil tanker
- Oil-bath
- Oil-vessel
- Oily
- Ointment
- Okay
- Old bachelor