- 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
- Old money
- Old salt
- Old school
- Old-fashioned
- Olden
- Oleaginous ness
- Oliver
- Omanization
- Omena
- Omissively
- Omit to
- Omniscience
- Omnium gatherum
- Omnivore
- On
- On active service
- On all counts
- On an even keel
- On demand system
- On duty
- On hand
- On its merits
- On ones high horse
- On ones legs
- On tenter hooks
- On the job