- old age
- old-age assistance
- old-age pensioner
- old as Methuselah
- old as the hills
- old bag
- old bat
- old battleaxe
- old biddy
- old-boy network
- old chestnut
- old college try
- old country
- old dame
- old days
- old dog
- olden
- olden days
- Old English
- olden times
- older
- oldest
- old faithful
- oldfangled
- old fart
- old-fashioned
- old fogey
- old fogy
- old-fogyish
- old folk
- old folks' home
- old folks home
- old geezer
- old girl
- old-girl network
- Old Glory
- old goat
- old grad
- old guard
- old hand
- old hat
- old heave-ho
- oldie
- oldish
- old lady
- old lang syne
- old-line
- old liner
- old maid
- old-maidish
- old man
- old master
- old money
- old moon
- oldness
- old one-two
- old people's home
- old person
- old poop
- old pro
- old prune
- old salt
- old saw
- old school
- old-school
- old school tie
- old sea dog
- old sod
- old softie
- old soldier
- old soldiers' home
- old stager
- oldster
- old story
- old-time
- old-time country rock
- old-timer
- old-time religion
- old times
- old town
- old war-horse
- old witch
- old wives' tale
- old woman
- old-world
- oleaginous
- oleo
- olfactive
- olfactory
- olfactory nerves
- olid
- oligarch
- oligarchic
- oligarchy
- oligopoly
- olio
- olive
- olive branch
- olive-brown
- olive drab
- olive-skinned
- olla
- Olympian
- Olympic lifting
- 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/off
- on account
- on account of
- on a day
- on a dime
- on a downer
- on-again, off-again
- on-again-off-again
- on ahead
- on a high horse
- on a hunch
- on a large scale
- on a leash
- on alert
- on a limb
- on a line
- on all accounts
- on all counts
- on all fours
- on a long shot
- on and off
- on and on
- on an ego trip
- on an even keel
- on any occasion
- on a par
- on approval
- on a roll
- on a slant
- on a small scale
- on a sour note
- on assignment
- on a streak
- on a string
- on-a-tear
- on a treadmill
- on a trip
- on average
- on bed of roses
- on behalf of
- on board
- on call
- once
- once a day
- once again
- once and again
- once and for all
- once a quarter
- once a year
- once daily
- once every three months
- once every two weeks
- once for all
- once in a blue moon
- once-in-a-lifetime
- once in awhile
- once in a while
- once in blue moon
- once more
- once only
- once or twice
- once-over
- once over lightly
- on certain conditions
- on certain occasions
- once upon a time
- on cloud nine
- on collision course
- oncoming
- on condition
- on credit
- on deck
- on display
- on dry land
- on duty
- one
- one's all
- one's best
- one's best guess
- one's choice
- one's country
- one's damndest
- one's darnedest
- one's discretion
- one's friend
- one's initiative
- one's level best
- one's move
- one's native soil
- one's own choice
- one's own sweet time
- one's own sweet way
- one's own will
- one's promised
- one's say
- one's space
- one's thing
- one's turn
- one accord
- one-act
- one after another
- one after the other
- one and all
- one and only
- one and the other
- one and the same
- one another
- one-armed bandit
- one at a time
- one at the controls
- one at the wheel
- one-bagger
- one by one
- one can assume
- one day
- on edge
- one-dimensional
- one-dog night
- one-eighty
- one fine day
- one foot in grave
- one foot in the grave
- one for the birds
- one for the book
- one for the books
- one for the road
- one-fourth
- one-gas-station town
- on ego trip
- one-horse town
- one hundred per cent
- one-hundred percent
- one in a million
- one into
- one-liner
- one man one vote
- one more
- one more time
- on empty stomach
- on end
- oneness
- one night engagement
- one-nighter
- one-night stand
- one of a kind
- one-of-a-kind
- one of the folks
- one of the herd
- one of these days
- one of two
- one-on-one
- one-party
- one-percenters
- one person at a time
- one-piece
- one-piece bathing suit
- one-piece suit
- oner
- onerous
- onerously
- oneself
- one-shot
- one-sided
- one-sidedness
- one-spot
- onest
- one-step
- one stoplight town
