- one thirty-sixth of a yard
- one-thousandth anniversary
- onetime
- one-time
- one time before
- one time or another
- one time previously
- one-track mind
- one-two
- one-two punch
- one-up
- one-upmanship
- one-upping
- one upsmanship
- one-way
- one way or another
- one-way ticket
- one who brings home the bacon
- one who is on the wagon
- one who signs the checks
- one you can hang the wash on
- on faith
- onfall
- on file
- on fire
- on foot
- on friendly terms
- on furlough
- ongoing
- 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
- onion skin
- on its way
- on land
- on last leg
- on layoff
- on leave
- onliest
- on-line
- Online banking
- online community
- online correspondence
- online demand publishing
- online mail
- online network
- online publishing
- onlooker
- onlookers
- on lookout
- only
- only a few
- only choice
- only just
- only one
- only one time
- only too happy
- on occasion
- on offer
- onomasticon
- onomastics
- onomatopoeia
- onomatopoeic
- onomatopoeical
- onomatopoetic
- 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 one occasion
- on one plane
- on one side
- on paper
- on pins and needles
- on probation
- on purpose
- on rare occasions
- on record
- onrush
- onrushing
- on sabbatical
- on safe side
- on sale
- on same wavelength
- on schedule
- on second thought
- onset
- onshore
- on shore
- onshore breeze
- on short notice
- on sick list
- onside kick
- on sidelines
- onslaught
- on slippery ground
- on someone
- on speaking terms
- on spur of moment
- on spur of the moment
- on stream
- on tap
- on-target
- on tenterhooks
- on that
- on that account
- on that occasion
- 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 Internet
- 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
- 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 Q.T.
- on the quiet
- on the qui vive
- 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 these terms
- on the shelf
- on the sick list
- on the side
- on the side of
- on the skids
- on the sly
- on-the-spot
- 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 thin ice
- on this spot
- on this subject
- on this wise
- on time
- onto
- on to
- on toes
- ontogenesis
- ontogeny
- ontological
- ontologism
- ontology
- 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
- onus
- onus probandi
- on vacation
- on view
- onward
- onward/onwards
- on way out
- on what
- on wheels
- 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-air theater
