oak
oars
oath
oats
obey the call of nature
occasion
occupy oneself
oddball
odd jobs
odds
odds and ends
odds are against
odds-on
of age
of a piece
of course
off a duck's back
off-again, on-again
off and on
off balance
off base
offbeat
off center
off-center
off-color
off day
off duty
off feed
off (one/he/she/it) goes!
off guard
offhand
off in a flash
off-key
off limits
off one's back
off one's chest
off one's feet
off one's hands
off one's head
off one's high horse
off one's nut
off one's rocker
off one's trolley
off season
offshoot
off the air
off the bat
off the beam
off the beaten track
off the cuff
off-the-cuff
off the ground
off the handle
off the hog
off the hook
off the record(1)
off the record(2)
off the top of one's head
off the wagon
off the wall
of it
of late
of necessity
of no avail
of old(1)
of old(2)
of oneself
of one's life
of one's own accord
of one's own free will
of service
of sorts
often
of die devil
of the first water
of the same mind
oil
oil the wheels
ointment
old
old as the hills
old boy network
old boy
old college try
old country
old flame
old guard
old hand
old hat
old maid
old money
Old Nick
old school tie
old story
old-timer
old world
olive branch
on account
on account of
on a dime
on-again, off-again
on air
on a limb
on all four
on all fours
on an average
on and off
on an even keel
on a pedestal
on approval
on a scratched horse
on a shoestring
on a volcano
on behalf of
on board(1)
on board(2)
on borrowed time
on call
once
once and for all
once bitten, twice shy
once for all
once in a blue moon
once in a while
once-over
once upon a time
on cloud nine
on condition that
on deck
on deposit
on duty
one
one and the same
one-armed bandit
on earth
on easy street
one damn thing after another
on edge
one eye on
one foot in the grave
one for the books
one-horse
one man's meat is another man's poison
on end
one-night stand
one of these days
one on the city
one's money's worth
one's own row
one-two
one up
one-upmanship
on faith
on file
on fire
on foot
on guard
on hand
on high cotton
on hold
on ice
on in years
onion
only
on occasion
on one hand
on one's account
on one's back
on one's bad side
on one's behalf
on one's chest
on one's coattails
on oneself
on one's feet
on one's good behavior
on one's good side
on one's hands
on one's head
on one's heel
on one's high horse
on one's honor
on one's knees
on one's last legs
on one's mind
on one's nerves
on one's own
on one's own account
on one's own feet
on one's own hook
on one's own time
on one's part
on one's shoulders
on one's toes
on one's uppers
on one's way
on pain of
on paper
on pins and needles
on purpose
on record
on sale
on schedule(1)
on schedule(2)
on season
on second thought
on sight
on table
on the air
on the alert
on the average
on the back
on the ball
on the bandwagon
on the barrelhead
on the beam
on the bench
on the blink
on the block
on the brain
on the brink of
on the button
on the cards
on the carpet
on the cheap
on the chin
on the contrary
on the cuff
on the dole
on the dot
on the double!
on the eve of
on the face of it
on the fence
on the fly(1)
on the fly(2)
on the go
on the heels of
on the hog
on the hour
on the house
on the increase
on the job
on the knees of the gods
on the lam
on the level
on the line
on the lips
on the lookout
on the loose
on the make
on the map
on the market
on the mend
on die money
on the move
on the nose
on the other foot
on the other hand
on the outs
on the point of
on the Q.T.
on the road
on the rocks
on the ropes
on the run
on the safe side
on the shelf
on the side
on the sly
on the spot
on the spur of the moment
on the stage
on the strength of
on the string
on the take
on the tip of one's tongue
on the town
on the track of
on the trail of
on the trigger
on the up and up
on the vine
on the wagon
on the wane and on the wax
on the warpath
on the watch
on the way
on the whole
on the wing
on the words
on the wrong foot
on the wrong side of the bed
on the wrong track
on time
on top
on top of
on top of the world
on trial
on words
on your life
open
open and aboveboard
open and shut
open arms
open-door policy
open fire
openhanded
open heart
open its doors
open letter
open market
open marriage
open-minded
open one's eyes
open one's heart
open onto
open Pandora's box
open question
open secret
open sesame
open shop
open the door
open the floodgates
open up
open up one's eyes
opinion
opposite number
opposite sex
optional origin
orbit
order
order about
ordinary
or other
or so
other
other fish to fry
other side of the tracks
out and about
out-and-out
out at the elbows
out back
outback
out cold
outer space
out for
out from under
out in force
out in left field
out in the cold
out in the open
out like a light
out loud
out of
out of a bandbox
out of account
out of a clear sky
out of action
out of all proportion
out of bounds
out of breath
out of character
out of circulation
out of commission
out of condition
out of date
out of fashion
out of gas
out of gear
out of hand
out of hot water
out of keeping
out of kilter
out of line(1)
out of line(2)
out of line with
out of luck
out of mind
out of nowhere
out of one's blood
out of one's element
out of one's hair
out of one's hand
out of one's head
out of one's mind
out of one's mouth
out of one's pocket
out of one's sails
out of one's senses
out of one's shell
out of one's skin
out of one's way
out of one's wits
out of (one's) reach
out of order
out of place(1)
out of place(2)
out-of-pocket expenses
out of practice
out of print
out of school
out of season
out of shape
out of sight
out of sight, out of mind
out of sorts
out of step
out of stock
out of the blue
out of the corner of one's eye
out of the frying pan into the fire
out of the hole
out of the ordinary
out of the picture
out of the question
out of the rain
out of the red
out of the running
out of the swim
out of the way
out of the woods
out of thin air
out of this world
out of touch
out of town
out of tune
out of turn
out of wedlock
out of whack
out of work
out on a limb
out on bail
out on parole
out on the town
outside of
out to lunch
overall
over a barrel
over age
over and done with
overboard
overhead
overnight
over one's dead body
over one's head
over spilled milk
over the coals
over the hill
over the hump
over the long haul
over the short haul
over the top
over the traces
over with(1)
over with(2)
own
own up
oyster