old as the hills
   odd man out
   odour of sanctity
   of all the nerve
   Of all things
   of benefit
   of late
   of no avail
   of one's own accord
   of one's own free will
   of the first water
   of the old school
   off again, on again
   off colour
   off duty
   off limits
   off one's nut
   off one's oats
   off one's rocker
   off one's trolley
   off someone goes
   off the air
   off the beaten track
   off the cuff
   off the record
   off the top of one's head
   off the wall
   off to a running start
   oh, yeah
   oil someone's palm
   oil the wheels
   old enough to be someone's father
   old enough to be someone's mother
   old hand at do
   on a diet
   on a first-name basis
   on a par with
   on account
   on account of
   on active duty
   on again, off again
   on all fours
   on appro
   on approval
   on average
   on behalf of
   on board
   on call
   on cloud nine
   on deck
   on deposit
   on earth
   on edge
   on foot
   on good terms
   on hold
   on holiday
   on in years
   on no account
   on one's own head be it
   on one's best behaviour
   on one's feet
   on one's guard
   on one's honour
   on one's mind
   on one's own
   on one's toes
   on one's way
   on order
   on pins and needles
   on record
   on sale
   on schedule
   on second thoughts
   on someone's last legs
   on someone's account
   on someone's back
   on someone's behalf
   on someone's doorstep
   on someone's say-so
   on someone's shoulders
   on speaking terms
   on target
   on the air
   on the alert
   on the ball
   on the beam
   on the bench
   on the blink
   on the button
   on the cards
   on the contrary
   on the dot
   on the double
   on the eve of
   on the face of it
   on the go
   on the horizon
   on the horns of a dilemma
   on the hot seat
   on the hour
   on the house
   on the job
   on the level
   on the look-out
   on the loose
   on the market
   on the mend
   on the move
   on the nose
   on the off chance
   on the one hand
   on the other hand
   on the point of
   on the q.t.
   on the sly
   on the spur of the moment
   on the strength of
   on the telephone
   on the track of
   on the trail of
   on the verge of
   on the wagon
   on the waiting-list
   on the war-path
   on the watch for
   on the way
   on the whole
   on the wing
   on the wrong track
   on tiptoe
   on top
   on top of
   on top of the world
   on trial
   on view
   once-in-a-lifetime chance
   one and all
   one and only
   one and the same
   one at a time
   one by one
   one first-name terms
   one for the record
   One good turn deserves another
   one in a hundred
   one in a million
   one in a thousand
   one in the eye for
   one jump ahead
   one means business
   one move ahead
   one of these days
   one way or another
   One's bark is worse than one's bite
   one's better half
   one's days are numbered
   one's eyes are bigger than one's stomach
   One's heart goes out to someone
   One's heart is in one's mouth
   one's heart is in the right place
   one's heart is set on
   one's heart misses a beat
   one's heart skips a beat
   one's heart stands still
   one's luck runs out
   one's name is mud
   one's note is in the air
   one's number is up
   one's tail is between one's legs
   one's way of life
   one's words stick in one's throat
   one's work is cut out
   one-track mind
   only have eyes for
   open a can of worms
   open and above-board
   open book
   open fire
   open one's heart
   open Pandora's box
   open season for
   open secret
   open someone's eyes
   open the door to
   opposite sex
   order of the day
   other way round
   out and about
   out cold
   out from under
   out in force
   out like a light
   out of all proportion
   out of bounds
   out of breath
   out of character
   out of circulation
   out of commission
   out of condition
   out of consideration
   out of control
   out of courtesy
   out of date
   out of fashion
   out of favour
   out of hand
   out of it
   out of keeping with
   out of kilter
   out of line
   out of luck
   out of necessity
   out of one's element
   out of one's head
   out of one's mind
   out of one's way
   out of place
   out of practice
   out of print
   out of reach
   out of season
   out of service
   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 ordinary
   out of the question
   out of the running
   out of the swim of things
   out of the way
   out of the woods
   out of the world
   out of time
   out of touch
   out of town
   out of tune
   out of turn
   out of work
   out on a limb
   out on ball
   out on parole
   out on the town
   out to lunch
   out-of-pocket expense
   over and above
   over and done with
   over and over
   over my dead body
   over the hill
   over the hump
   over the odds
   over the top
   object against
   object to
   oblige by
   oblige to
   oblige with
   observe on
   obsess with
   obtain for
   obtain from
   obtrude on
   occupy in
   occupy with
   occur to
   offend against
   offend with
   offer for
   offer to
   offer up
   officiate as
   officiate at
   ogle at
   omit from
   one's blood be up
   one's blood run cold
   one's luck be in
   ooze away
   ooze out
   open fire on
   open into
   open off
   open on
   open onto
   open out
   open to
   open up
   open with
   operate against
   operate from
   operate on
   oppose to
   opt for
   opt in
   opt out
   order about
   order from
   order in
   order off
   order out
   order up
   orientate oneself
   originate from
   originate in
   originate with
   ornament with
   oscillate about
   oscillate between
   oust from
   overcrowd with
   overflow with
   overreach oneself
   owe for
   owe it to
   owe to
   own to
   own up
   O level
   oak apple
   oak tree
   obiter dicta
   obiter dictum
   object ball
   object glass
   object lens
   object lesson
   objective case
   objet d'art
   observation car
   obstacle course
   obstacle race
   occasional table
   occult science
   occupational disease
   occupational therapist
   occupational therapy
   ocean greyhound
   ocean liner
   octane number
   Odd Fellow
   odd jobs
   odd-job man
   odds and ends
   Oedipus complex
   off and on
   off chance
   off guard
   off season
   office block
   office boy
   office girl
   office hours
   office worker
   official receiver
   offset press
   Ohm's law
   oil color
   oil colour
   oil field
   oil paint
   oil painting
   oil pan
   oil rig
   oil shale
   oil slick
   oil tanker
   oil well
   oilseed rape
   old age
   old bat
   Old Bill
   old boy
   old country
   Old English
   Old English sheepdog
   old flame
   Old French
   old girl
   Old Glory
   old guard
   old hand
   old hat
   old lady
   old lag
   old maid
   old man
   old master
   Old Nick
   Old Norse
   old people's home
   old stager
   Old Testament
   old woman
   Old World
   old-age pension
   old-age pensioner
   olde worlde
   olive branch
   olive oil
   Oliver Cromwell
   Olympic Games
   Omar Khayyam
   on and off
   on duty
   on guard
   on occasion
   on the spot
   on time
   once and for all
   once in a blue moon
   once or twice
   once upon a time
   one another
   one more
   one too many
   one-armed bandit
   one-man band
   one-night stand
   one-parent family
   one-way street
   one-way traffic
   onion skin
   only child
   op art
   op. cit.
   open day
   open house
   open letter
   open market
   open marriage
   open prison
   open season
   open sesame
   open town
   open verdict
   open-and-shut case
   open-heart surgery
   opening hours
   opening night
   opening time
   opening up
   open work
   opera glasses
   opera hat
   operating room
   operating system
   operating table
   operating theatre
   operational research
   operations research
   ophthalmic optician
   opinion poll
   opium poppy
   opposites attract
   optical character recognition
   optical disc
   optical disk
   optical fiber
   optical fibre
   optical illusion
   or else
   or so
   oral tradition
   orange squash
   orchestral pit
   order book
   order form
   Order Paper
   ordinal number
   ordinary level
   ordinary share
   Ordnance Survey map
   organ loft
   organic chemistry
   original sin
   orlop deck
   Orthodox Church
   osmotic pressure
   other than
   other woman
   other world
   Ottoman Empire
   ought to
   Ouija board
   Our Lady
   out of
   out of order
   out tray
   outboard engine
   outboard motor
   outer belt
   outer space
   outside broadcast
   outward bound
   oven glove
   oven mitt
   over again
   over against
   over and over again
   over easy
   over the counter
   over the way
   overall majority
   overhead camshaft
   overhead projector
   overhead railway
   overly much
   overt act
   owing to
   own goal
   oxalic acid
   Oxford accent
   Oxford blue
   Oxford English
   oxidizing agent
   oxygen mask
   oxygen tent
   Oh, boy
   old fogey
   old heave-ho
   old soldier
   oldie but goodie
   on a tank
   on a tight leash
   on tap
   on the fritz
   on the juice
   on the make
   on the pill
   on the rocks
   on the sauce
   on the squiff
   on the street
   on the take
   On your bike
   once over lightly
   one for the road
   one-man show
   only way to go
   out the gazoo
   odour colon
   off your face
   off your rocker
   off your trolley
   oily rag
   old chap
   old fruit
   on the game
   On your Todd
   out of your tree