| on the table |
| on the up-and-up |
| on the warpath |
| on the wrong side of the blanket |
| on top of |
| onwards and upwards |
| oof-bird |
| open-collar |
| open fire |
| open one's mind |
| open someone's eyes |
| open the kimono |
| opposite number |
| orgiast |
| ostrich policy |
| other head |
| otherwise engaged |
| out cold |
| out of bounds |
| out of date |
| out of house and home |
| out of luck |
| out of one's face |
| out of order |
| out of shape |
| out of someone's way |
| out of the frying pan, into the fire |
| out of the ordinary |
| out of the woods |
| out of touch |
| out on the tiles |
| outrun the constable |
| outside job |
| over a barrel |
| overegg |
| over one's head |
| over the hump |
| over the top |
| odd duck |
| of a |
| of all people |
| of a piece |
| off board |
| offer one can't refuse |
| off-kilter |
| off one's feed |
| off one's nut |
| off one's trolley |
| off the bat |
| off the deep end |
| off the radar |
| off-the-shelf |
| off-the-wall |
| of sorts |
| of the first water |
| oh dark hundred |
| oh my goodness gracious |
| oil burner |
| old chestnut |
| old flame |
| old head on young shoulders |
| old school |
| old wine in a new bottle |
| on a day-to-day basis |
| on a kick |
| on and off |
| on a plate |
| on a sixpence |
| on bad terms |
| once again |
| once in a purple moon |
| on cloud seven |
| one age with |
| one and the same |
| one could do it in one's sleep |
| one flesh |
| one-horse town |
| one-man band |
| one-night stand |
| one of those things |
| one's blood is up |
| one's heart in one's mouth |
| onesie-twosie |
| one's name is mud |
| one step ahead |
| on eternal patrol |
| one-track mind |
| on file |
| on hold |
| only game in town |
| on one's feet |
| on one's mind |
| on opposite sides of the barricades |
| on second thought |
| on someone's dime |
| on someone's radar |
| on sufferance |
| on the back burner |
| on the beach |
| on the breadline |
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