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O Romeo, Romeo! wherefore art thou Romeo?
O ye, of little faith
Odds bodkins
Off the record
Off with his head
Oh, they have slain the Earl of Moray and Lady Mondegreen
Okay
Okey-dokey
Old as Methuselah
Old as the hills
Old lang syne - see auld lang syne
On a hiding to nothing
On a wing and a prayer
On Carey Street
On cloud nine 
On queer street
On the bubble
On the dole
On the QT
On the shoulders of giants
On the wagon
On the warpath
On with the motley
On your beam ends
On your tod
Once in a blue moon
Once more unto the breach, dear friends, once more
One-hit wonder
One for the road
One over the eight
One sandwich short of a picnic
One small step for man
One stop shop
One's heart's content
Oops-a-daisy
Open season
(The) order of the boot
Out of sight, out of mind
Out of sorts
Out of the jaws of death
Out of the mouths of babes and sucklings
Over a barrel
Over the moon
Over the top
Overpaid, oversexed, and over here
Oxo cube