O Romeo, Romeo! wherefore art thou Romeo?

O ye, of little faith

Odds bodkins

Off his own bat

Off the record

Off with his head

Offing - In the

Oh, my stars and garters

Oh, they have slain the Earl of Moray and Lady Mondegreen

Okay

Okey-dokey

Old as Methuselah

Old as the hills

Old chestnut

Old codger

Old hat

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 tenterhooks

On the ball

On the bubble

On the button

On the dole

On the fiddle

On the pig's back

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 fell swoop - At

One for the road

One over the eight

One sandwich short of a picnic

One small step for man

One stop shop

One swallow doesn't make a summer

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