- on the nail
- on the nether side
- on the next day
- on the nose
- on the numbers
- on the occasion that
- on the order of
- on the other hand
- on the other side
- on the outs
- on the outside
- on the part of
- on the payroll
- on the point
- on the point of
- on the qui vive
- on the quiet
- on the rampage
- on the right track
- on the rims
- on the road
- on the road to
- on the road to recovery
- on the rocks
- on the ropes
- on the same wavelength
- on the scent
- on the shelf
- on the sick list
- on the side
- on the side of
- on the skids
- on the sly
- on the spur of the moment
- on the stick
- on the subject of
- on the surface
- on the tab
- on the take
- on the underside
- on the up and up
- on the verge
- on the verge of
- on the wagon
- on the warpath
- on the watch
- on the way
- on the way to
- on the whole
- on the wing
- on the wrong track
- on these terms
- on thin ice
- on this spot
- on this subject
- on this wise
- on time
- on to
- on toes
- on top
- on top of
- on top of each other
- on top of everything
- on top of heap
- on top of the heap
- on top of the world
- on top of world
- on tour
- on track
- on treadmill
- on trial
- on vacation
- on view
- on way out
- on what
- on wheels
- on-a-tear
- on-again, off-again
- on-again-off-again
- on-line
- on-target
- on-the-spot
- on/off
- once
- once a day
- once a quarter
- once a year
- once again
- once and again
- once and for all
- once daily
- once every three months
- once every two weeks
- once for all
- once in a blue moon
- once in a while
- once in awhile
- once in blue moon
- once more
- once only
- once or twice
- once over lightly
- once upon a time
- once-in-a-lifetime
- once-over
- oncoming
- one
- one accord
- one after another
- one after the other
- one and all
- one and only
- one and the other
- one and the same
- one another
- one at a time
- one at the controls
- one at the wheel
- one by one
- one can assume
- one day
- 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 hundred per cent
- one in a million
- one into
- one man one vote
- one more
- one more time
- one night engagement
- one of a kind
- one of the folks
- one of the herd
- one of these days
- one of two
- one person at a time
- one stoplight town
- one thirty-sixth of a yard
- one time before
- one time or another
- one time previously
- one upsmanship
- one way or another
- one who brings home the bacon
- one who is on the wagon
- one who signs the checks
- one you can hang the wash on
- 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-act
- one-armed bandit
- one-bagger
- one-dimensional
- one-dog night
- one-eighty
- one-fourth
- one-gas-station town
- one-horse town
- one-hundred percent
- one-liner
- one-night stand
- one-nighter
- one-of-a-kind
- one-on-one
- one-party
- one-percenters
- one-piece
- one-piece bathing suit
- one-piece suit
- one-shot
- one-sided
- one-sidedness
- one-spot
- one-step
- one-thousandth anniversary
- one-time
- one-track mind
- one-two
- one-two punch
- one-up
- one-upmanship
- one-upping
- one-way
- one-way ticket
- oneness
- oner
- onerous
- onerously
- oneself
- onest
- onetime
- onfall
- ongoing
- onion skin
- onliest
- online community
- online correspondence
- online demand publishing
- online mail
- online network
- online publishing
- onlooker
- onlookers
- only
- only a few
- only choice
- only just
- only one
- only one time
- only too happy
- onomasticon
- onomastics
- onomatopoeia
- onomatopoeic
- onomatopoeical
- onomatopoetic
- onrush
- onrushing
- onset
- onshore
- onshore breeze
- onside kick
- onslaught
- onto
- ontogenesis
- ontogeny
- ontological
- ontologism
- ontology
- onus
- onus probandi
- onward
- onward/onwards
- onyx
- oodles
- ooid
- oomph
- oops
- oops-a-daisy
- oosphere
- oospore
- ootheca
- ooze
- oozed
- oozing
- oozy
- opacity
- opalescence
- opalescent
- opaline
- opaque
- opaqueness
- open
- open account
- open air
- open and aboveboard
- open and shut case/open-and-shut case
- open and shut/open-and-shut
- open arms
- open can of worms
- open country
- open discussion
- open door
- open doors
- open doors for
- open eyes
- open fire
- open for consideration
- open gate
- open hostilities
- open into
- open market
- open marriage
- open ocean
- open one's eyes