- one-parent family
- one picture is worth a thousand words
- one-piece
- oner
- one-reeler
- One rotten (or bad) apple spoils the barrel
- onerous
- oneself
- one-shot
- one-sided
- onesie
- one-size-fits-all
- one-spot
- one-star
- one-step
- one-stop
- one-suiter
- one-tailed
- one that loved not wisely but too well
- One Thousand Guineas
- one-time
- one-to-one
- one-track
- one-track mind
- one-trick pony
- Onetti
- one-two
- one up
- one-up
- one-upmanship
- one-way
- one-way mirror
- one way or another
- one-way ticket
- one-woman
- one-worlder
- oneˈ-upmanship
- on faith, take it
- on file
- on fire
- on foot
- ONFr.
- ongaonga
- on-glide
- ongoing
- ongoings
- on good terms
- on hand
- on high
- on hold
- ONI
- on ice
- onie
- onigiri
- on in years
- oniomania
- onion
- onion dome
- onion fly
- onion rings
- Onions
- onionskin
- onion snow
- onion weed
- Onitsha
- on its merits
- onium compound
- Onkelos
- on key
- onkus
- onlap
- onlay
- On Liberty
- onliest
- on-limits
- online
- on line
- online catalog
- online dictionary
- onliner
- online thesaurus
- onload
- onlooker
- onlooking
- only
- only-begotten
- only game in town, the
- only too
- on message
- on-message
- on-mike
- onnagata
- on no account
- on occasion
- on-off
- onomasiology
- onomastic
- onomasticon
- onomastics
- onomatology
- onomatopoeia
- Onondaga
- Onondagan
- on one's
- on one's account
- on one's behalf
- on one's best behavior
- on one's doorstep
- on one's feet
- on one's good behavior
- on one's guard
- on one's head
- on one's heels
- on one's high horse
- on one's home ground
- on one's honor
- on one's last legs
- on one's mind
- on one's own
- on one's own account
- on one's own time
- on one's part
- on one's say-so
- on one's shoulders
- on one's soapbox
- on one's tail
- on one's toes
- on one's uppers
- on one's way
- on one hand
- on oneself
- on order
- on pain of
- on paper
- on-peak
- on pins and needles
- on principle
- on purpose
- ONR
- on-ramp
- on record
- on-record
- on relief
- on request
- onrush
- ONS
- Onsager
- on sale
- on schedule
- on-screen
- on-seam
- on-season
- on second thought
- onset
- onshore
- onshoring
- onside
- onside kick
- on sight
- on-site
- onslaught
- Onslow
- on someone's
- on someone's back
- on someone's coattails
- on someone's nerves
- on someone's side
- on speaking terms
- on spec
- onstage
- on standby
- on-stream
- on strike
- on sufferance
- ont-
- Ont.
- on tap
- on target
- on-target
- Ontarian
- Ontario
- on tenterhooks
- on the air
- on the alert
- on the average
- on the ball, be
- on the bandwagon, get
- on the barrel
- on the beam
- on the bench
- on the blink
- on the block
- on the brain
- on the bum
- on the button
- on the carpet
- on the chance that
- on the cheap
- on the chin
- on the contrary
- on the cuff
- on the defensive
- on the dot
- on the double
- on the edge
- on the eve of
- on the face of it
- on the fence, be
- on the fly
- on the fritz
- on the go
- on the heels of
- on the Hill
- on the horizon
- on the horns of a dilemma
- on the hour
- on the house
- on the in, be
- on the increase
- on the inside
- on the job
- on-the-job
- on the lam
- on the level
- on the line
- on the lines of
- on the lookout
- on the loose
- on the make
- on the map
- on the mark
- on the market
- on the mend
- on the money
- on the move
- on the nail
- on the nose
- on the off chance
- on the one hand
- on the order of
- on the other foot
- on the other hand
- on the outs
- on the part of
- on the point of
- on the prowl
- on the Q.T.
- on the qui vive
- on the rack
- on the rebound
- on the receiving end
- on-the-record
- on the right foot, get off
- on the right tack
- on the road
- On the Road
- on the rocks
- on the ropes
- on the run
- on the safe side
- on the same wavelength
- on-the-scene
- on the scene, be
- on the shelf
- on the side
- on the sidelines
- on the side of the angels
- on the skids
- on the sly
- on the spot
- on-the-spot
- on the spur of the moment
- on the square
- on the street
- on the strength of
- on the surface
- on the table
- on the take
- on the tip of one's tongue
- on the town
- on the up-and-up
- on the uptake
- on the verge of
- on the wagon
- on the warpath
- on the watch
- on the way
- on the way out
- on the whole
- on the wing
- on the wrong foot
- on the wrong side of bed
- on the wrong tack
- on thin ice
- ontic
- on time
- on tiptoe
- onto
- onto-
- on to
