- 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
- On the map
- On the move
- On the one hand
- On the other hand
- On the quiet
- On the record
- On the rise
- On the ropes
- On the safe side
- On the score of absurdity
- On the side
- On the subject of
- On the turn
- On the way
- On tow
- On-the-wing
- Once again
- Once and again
- Once in a blue moon
- Onyx
- Once upon a time
- Oncoming
- One
- One above
- One by one
- One day
- One hundred thousand
- One ideal
- One mans meat is another mans poison
- One of 20 parts
- One of the 8 elephants guarding the 8 corners of the universe
- One of the 9 planets
- One of the 9 sentiments
- One rate
- Optophone