| 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| out of date | | | |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 | | | |