| on the cards | | | |
| on the cutting room floor | | | |
| on the down-low | | | |
| on the edge of one's seat | | | |
| on the fly | | | |
| on the game | | | |
| on the high rope | | | |
| on the horn | | | |
| on the ladder | | | |
| on the loose | | | |
| on the move | | | |
| on the other side of | | | |
| on the plus side | | | |
| on the Q.T. | | | |
| on the rails | | | |
| on the right side of history | | | |
| on the same wavelength | | | |
| on the sly | | | |
| on the stocks | | | |
| on the trot | | | |
| on the wagon | | | |
| on the whole | | | |
| on this score | | | |
| on track | | | |
| on with you | | | |
| open-armed | | | |
| open doors | | | |
| open invitation | | | |
| open Pandora's box | | | |
| open the door | | | |
| open wide | | | |
| or else | | | |
| or what | | | |
| other fish in the sea | | | |
| other side | | | |
| our era | | | |
| out loud | | | |
| out of character | | | |
| out of fix | | | |
| out of harness | | | |
| out of nowhere | | | |
| out of one's element | | | |
| out of place | | | |
| out of season | | | |
| out of sorts | | | |
| out of the chute | | | |
| out of the picture | | | |
| out of the way | | | |
| out of wedlock | | | |
| out on one's feet | | | |
| out sick | | | |
| out the window | | | |
| over and out | | | |
| over nine thousand | | | |
| overstep the mark | | | |
| over the moon | | | |
| ox is in the ditch | | | |
| out of frame | | | |
| out of line | | | |
| out of one's box | | | |
| out of one's mind | | | |
| out of pocket | | | |
| out of sight, out of mind | | | |
| out of spoons | | | |
| out of the loop | | | |
| out of the question | | | |
| out of this world | | | |
| out of whack | | | |
| outpope the Pope | | | |
| outside the box | | | |
| out to lunch | | | |
| over my dead body | | | |
| over the hill | | | |
| over the line | | | |
| oxygen burning | | | |
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