| 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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