| oateater | | | |
| oat opera | | | |
| odd and curious | | | |
| odd duck | | | |
| odd fish | | | |
| odds and ends | | | |
| odds and sods | | | |
| of a | | | |
| of age | | | |
| of a kind | | | |
| of all loves | | | |
| of all people | | | |
| of all places | | | |
| of all things | | | |
| of an | | | |
| of a piece | | | |
| of choice | | | |
| off and running | | | |
| off balance | | | |
| off board | | | |
| off chance | | | |
| off-color | | | |
| offer affordances | | | |
| offer one can't refuse | | | |
| offer one's condolences | | | |
| offer up | | | |
| Officer Friendly | | | |
| off-kilter | | | |
| off one's block | | | |
| off one's box | | | |
| off one's dot | | | |
| off one's feed | | | |
| off one's game | | | |
| off one's hinge | | | |
| off one's hinges | | | |
| off one's nut | | | |
| off one's own bat | | | |
| off one's tits | | | |
| off one's tree | | | |
| off one's trolley | | | |
| off pat | | | |
| off-roader | | | |
| off the back foot | | | |
| off the bat | | | |
| off the beaten path | | | |
| off the beaten track | | | |
| off-the-cuff | | | |
| off the deep end | | | |
| off the face of the earth | | | |
| off the grid | | | |
| off the hook | | | |
| off the radar | | | |
| off the rails | | | |
| off the reservation | | | |
| off the scale | | | |
| off-the-shelf | | | |
| off the table | | | |
| off the top of one's head | | | |
| off the wagon | | | |
| off-the-wall | | | |
| off to the races | | | |
| of late | | | |
| of one mind | | | |
| of sorts | | | |
| of sound mind | | | |
| often wrong, never in doubt | | | |
| of the deepest dye | | | |
| of the first water | | | |
| of the same leaven | | | |
| of the same stripe | | | |
| of two minds | | | |
| oh dark hundred | | | |
| oh dark thirty | | | |
| oh my | | | |
| oh my goodness | | | |
| oh my goodness gracious | | | |
| oh my gosh | | | |
| oh well | | | |
| oil and water | | | |
| oil burner | | | |
| oil trash | | | |
| old as the hills | | | |
| old boy network | | | |
| old chestnut | | | |
| old enough to vote | | | |
| old fart | | | |
| old-fashioned look | | | |
| old flame | | | |
| old fogey | | | |
| old hand | | | |
| old hat | | | |
| old head on young shoulders | | | |
| old money | | | |
| old salt | | | |
| old saw | | | |
| old school | | | |
| old song | | | |
| old stager | | | |
| old stick | | | |
| old wine in a new bottle | | | |