- Octet rule
- Octo-
- Octopus
- Od
- Odd man
- Odd work
- Oddity
- Odiousness
- Odium
- Odontology
- Odoriferous
- Odorous
- Oeuvre
- Of a low standard
- Of an even temper
- Of gestures
- Of god's
- Of no effect
- Of note
- Of old standing
- Of one and one only
- Of ones own volition
- Of so little
- Of the left
- Of the moon's
- Of wide fame
- Of words
- Off duty
- Off line
- Off ones hand
- Off ones onion
- Off ramp
- Off scoring
- Off the point
- Off the track
- Off with his head
- Off-day
- Off-street
- Offensive
- Offerings
- Official dom
- Officially
- Officials
- Ogling
- Oh save!
- Ohm
- Oil colour
- Oil nuts
- Oil-skin
- Oiling
- Oily grain
- Olam
- Old age
- Old hand
- Old man
- Old rags
- Old stage
- Old story
- Old wares
- Old wine in a new bottle
- Old wisdom
- Old woman
- Older
- Oleander
- Oleograph
- Olfaction
- Ology
- Olympiad
- Olympus
- Omega
- Omissible
- Omit
- Omni
- Omni competent
- Omni volume
- Omnifarous
- Omniscient god
- Omnivorousness
- Omr
- On -dit
- On a par witih
- On an empty stomach
- On and off
- On board
- On cost
- On death bed
- On easy terms
- On fall
- Oneiric
- On fire
- On foot
- On impulse
- On me
- On occasion
- On one mettle
- On one side
- On ones heels
- On ones last legs
- On persons tail
- On request