| on the point of | | | |
| on the prowl | | | |
| on the pull | | | |
| on the Q.T. | | | |
| on the rack | | | |
| on the radar | | | |
| on the rag | | | |
| on the rails | | | |
| on the rampage | | | |
| on the ready | | | |
| on the receiving end | | | |
| on the right side of history | | | |
| on the rise | | | |
| on the run | | | |
| on the same page | | | |
| on the same wavelength | | | |
| on the shelf | | | |
| on the side of the angels | | | |
| on the skids | | | |
| on the sly | | | |
| on the spot | | | |
| on-the-spot | | | |
| on the square | | | |
| on the stocks | | | |
| on the street | | | |
| on the table | | | |
| on the toss of a coin | | | |
| on the trot | | | |
| on the up | | | |
| on the up-and-up | | | |
| on the uptake | | | |
| on the wagon | | | |
| on the wane | | | |
| on the warpath | | | |
| on the way | | | |
| on the whole | | | |
| on the wrong side of history | | | |
| on the wrong side of the blanket | | | |
| on thin ice | | | |
| on this score | | | |
| on top | | | |
| on top of | | | |
| on top of the world | | | |
| on track | | | |
| onward and upward | | | |
| onwards and upwards | | | |
| on what score | | | |
| on with you | | | |
| on yer bike | | | |
| oof-bird | | | |
| open a can of whoop ass | | | |
| open-armed | | | |
| open book | | | |
| open-collar | | | |
| open-door policy | | | |
| open doors | | | |
| open field | | | |
| open fire | | | |
| opening of an envelope | | | |
| open invitation | | | |
| open one's legs | | | |
| open one's mind | | | |
| open one's mouth | | | |
| open Pandora's box | | | |
| open season | | | |
| open someone's eyes | | | |
| open the batting | | | |
| open the door | | | |
| open the floodgates | | | |
| open the kimono | | | |
| open the schools | | | |
| open wide | | | |
| opinions are like assholes | | | |
| opposite number | | | |
| oranges with oranges | | | |
| or else | | | |
| Oreo cookie | | | |
| orgiast | | | |
| or something | | | |
| or what | | | |
| or words to that effect | | | |
| ostrich policy | | | |
| other end of the ball | | | |
| other fish in the sea | | | |
| other half | | | |
| other head | | | |
| other man | | | |
| other side | | | |
| other than | | | |
| otherwise engaged | | | |
| other woman | | | |
| our era | | | |
| out and about | | | |
| out cold | | | |
| out for blood | | | |
| out loud | | | |
| out of a nightmare | | | |
| out of bounds | | | |
| out of central casting | | | |
| out of character | | | |