| on fire |
| on hand |
| on ice |
| on its merits |
| only time will tell |
| on one's deathbed |
| on one's high horse |
| on one's lonesome |
| on one's tod |
| on one's watch |
| on pins and needles |
| on-ramp |
| on sight |
| on someone's account |
| on someone's doorstep |
| on someone's mind |
| on steroids |
| on stun |
| on talking terms |
| on the anvil |
| on the back foot |
| on the ball |
| on the boob |
| on the brain |
| on the bubble |
| on the canvas |
| on the clock |
| on the cusp |
| on the debit side |
| on the double |
| on the drum |
| on the edge |
| on the face of |
| on the flip side |
| on the fritz |
| on the front foot |
| on the go |
| on the heels of |
| on the high ropes |
| on the hook |
| on the horns of a dilemma |
| on the hush |
| on the lam |
| on the lines of |
| on the mark |
| on the money |
| on the nose |
| on the other hand |
| on the outs |
| on the pill |
| on the point of |
| on the pull |
| on the rack |
| on the rag |
| on the rampage |
| on the receiving end |
| on the rise |
| on the same page |
| on the shelf |
| on the skids |
| on the spot |
| on the square |
| on the street |
| on the toss of a coin |
| on the up |
| on the uptake |
| on the wane |
| on the way |
| on the wrong side of history |
| on thin ice |
| on top |
| on top of the world |
| onward and upward |
| on what score |
| on yer bike |
| open a can of whoop ass |
| open book |
| open-door policy |
| open field |
| opening of an envelope |
| open one's legs |
| open one's mouth |
| open season |
| open the batting |
| open the floodgates |
| open the schools |
| opinions are like assholes |
| oranges with oranges |
| Oreo cookie |
| or something |
| or words to that effect |
| other end of the ball |
| other half |
| other man |
| other than |
| other woman |
| out and about |
| out for blood |
| out of a nightmare |
| out of central casting |
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