| out of collar | | | |
| out of gas | | | |
| out of kilter | | | |
| out of one's depth | | | |
| out of one's league | | | |
| out of proportion | | | |
| out of sight | | | |
| out of the blue | | | |
| out of the gate | | | |
| out of the running | | | |
| out of thin air | | | |
| out on a limb | | | |
| out-paramour the Turk | | | |
| outside chance | | | |
| outside world | | | |
| over and over again | | | |
| over one's skis | | | |
| over the hills and far away | | | |
| over the transom | | | |
| oat opera | | | |
| odds and ends | | | |
| of a kind | | | |
| of all things | | | |
| off and running | | | |
| off-color | | | |
| offer up | | | |
| off one's box | | | |
| off one's hinge | | | |
| off one's tits | | | |
| off-roader | | | |
| off the beaten track | | | |
| off the grid | | | |
| off the reservation | | | |
| off the top of one's head | | | |
| of late | | | |
| often wrong, never in doubt | | | |
| of the same stripe | | | |
| oh my | | | |
| oh well | | | |
| old as the hills | | | |
| old fart | | | |
| old hand | | | |
| old salt | | | |
| old stager | | | |
| on account of | | | |
| on a first-name basis | | | |
| on all fours | | | |
| on an irregular basis | | | |
| on a shoestring | | | |
| on average | | | |
| on board | | | |
| once bitten, twice shy | | | |
| once or twice | | | |
| on demand | | | |
| one-and-done | | | |
| one big happy family | | | |
| on edge | | | |
| one-hit wonder | | | |
| one in the eye for | | | |
| one more again | | | |
| one-off | | | |
| one's back is up | | | |
| one's days are numbered | | | |
| one's hour | | | |
| one's lips are sealed | | | |
| one's socks off | | | |
| one step forward, two steps back | | | |
| one thing or another | | | |
| one up | | | |
| on good terms | | | |
| on in years | | | |
| on one's bill | | | |
| on one's last legs | | | |
| on one's toes | | | |
| on purpose | | | |
| on solid ground | | | |
| on someone's hands | | | |
| on strike | | | |
| on tenterhooks | | | |
| on the back of | | | |
| on the bounce | | | |
| on the button | | | |
| on the cuff | | | |
| on the dot | | | |
| on the dry | | | |
| on the face of it | | | |
| on the front burner | | | |
| on the gripping hand | | | |
| on the hoof | | | |
| on the house | | | |
| on the level | | | |
| on the mat | | | |
| on the one hand | | | |
| on the outside, looking in | | | |
| on the prowl | | | |
| on the radar | | | |
| on the ready | | | |
| on the run | | | |
| on the side of the angels | | | |
| on-the-spot | | | |