- old fart
- old fogey
- old fogy
- old folk
- old folks home
- old folks' home
- old geezer
- old girl
- old goat
- old grad
- old guard
- old hand
- old hat
- old heave-ho
- old lady
- old lang syne
- old liner
- old maid
- old man
- old master
- old money
- old moon
- old one-two
- old people's home
- old person
- old poop
- old pro
- old prune
- old salt
- old saw
- old school
- old school tie
- old sea dog
- old sod
- old softie
- old soldier
- old soldiers' home
- old stager
- old story
- old times
- old town
- old war-horse
- old witch
- old wives' tale
- old woman
- old-age assistance
- old-age pensioner
- old-boy network
- old-fashioned
- old-fogyish
- old-girl network
- old-line
- old-maidish
- old-school
- old-time
- old-time country rock
- old-time religion
- old-timer
- old-world
- olden
- olden days
- olden times
- older
- oldest
- oldfangled
- oldie
- oldish
- oldness
- oldster
- oleaginous
- oleo
- olfactive
- olfactory
- olfactory nerves
- olid
- oligarch
- oligarchic
- oligarchy
- oligopoly
- olio
- olive
- olive branch
- olive drab
- olive-brown
- olive-skinned
- olla
- ombre
- ombudsman
- omega
- omen
- omens
- omination
- ominous
- omission
- omit
- omitted
- omitting
- omneity
- omnibus
- omnifarious
- omnipotence
- omnipotent
- omnipresence
- omnipresent
- omniscient
- omnitude
- omnium gatherum
- omnivore
- omnivorous
- omnivorousness
- omophagous
- omphalic
- omphalos
- on
- on a day
- on a dime
- on a downer
- on a high horse
- on a hunch
- on a large scale
- on a leash
- on a limb
- on a line
- on a long shot
- on a par
- on a roll
- on a slant
- on a small scale
- on a sour note
- on a streak
- on a string
- on a treadmill
- on a trip
- on account
- on account of
- on ahead
- on alert
- on all accounts
- on all counts
- on all fours
- on an ego trip
- on an even keel
- on and off
- on and on
- on any occasion
- on approval
- on assignment
- on average
- on bed of roses
- on behalf of
- on board
- on call
- on certain conditions
- on certain occasions
- on cloud nine
- on collision course
- on condition
- on credit
- on deck
- on display
- on dry land
- on duty
- on edge
- on ego trip
- on empty stomach
- on end
- on faith
- on file
- on fire
- on foot
- on friendly terms
- on furlough
- on good terms
- on guard
- on hand
- on hands and knees
- on head
- on high
- on high horse
- on hold
- on holiday
- on ice
- on impulse
- on its way
- on land
- on last leg
- on layoff
- on leave
- on lookout
- on occasion
- on offer
- on one occasion
- on one plane
- on one side
- on one's deathbed
- on one's game
- on one's guard
- on one's head
- on one's high horse
- on one's knees
- on one's last leg
- on one's own
- on one's own initiative
- on one's own time
- on one's side
- on one's toes
- on one's uppers
- on paper
- on pins and needles
- on probation
- on purpose
- on rare occasions
- on record
- on sabbatical
- on safe side
- on sale
- on same wavelength
- on schedule
- on second thought
- on shore
- on short notice
- on sick list
- on sidelines
- on slippery ground
- on someone
- on speaking terms
- on spur of moment
- on spur of the moment
- on stream
- on tap
- on tenterhooks
- on that
- on that account
- on that occasion
- on the Internet
- on the Q.T.
- on the alert
- on the arm
- on the assumption
- on the assumption that
- on the back of
- on the ball
- on the beam
- on the beat
- on the bench
- on the bias
- on the blink
- on the borderline
- on the bottom
- on the brain
- on the breadline
- on the brink
- on the brink of
- on the bum
- on the button
- on the canvas
- on the case
- on the comeback trail
- on the contrary
- on the critical list
- on the cuff
- on the cutting edge
- on the debit side
- on the defensive
- on the docket
- on the dole
- on the dot
- on the double
- on the east side of
- on the edge
- on the edge of
- on the face
- on the face of it
- on the far side
- on the fence
- on the fritz
- on the front burner
- on the go
- on the grounds
- on the grounds that
- on the hit list
- on the hook
- on the horizon
- on the hot seat
- on the house
- on the inside
- on the job
- on the kip
- on the lam
- on the left
- on the level
- on the line
- on the lookout
- on the lookout for
- on the loose
- on the make
- on the mark
- on the mend
- on the money
- on the move