| oateater | | | |
| odd and curious | | | |
| odd fish | | | |
| odds and sods | | | |
| of age | | | |
| of all loves | | | |
| of all places | | | |
| of an | | | |
| of choice | | | |
| off balance | | | |
| off chance | | | |
| offer affordances | | | |
| offer one's condolences | | | |
| Officer Friendly | | | |
| off one's block | | | |
| off one's dot | | | |
| off one's game | | | |
| off one's hinges | | | |
| off one's own bat | | | |
| off one's tree | | | |
| off pat | | | |
| off the back foot | | | |
| off the beaten path | | | |
| off-the-cuff | | | |
| off the face of the earth | | | |
| off the hook | | | |
| off the rails | | | |
| off the scale | | | |
| off the table | | | |
| off the wagon | | | |
| off to the races | | | |
| of one mind | | | |
| of sound mind | | | |
| of the deepest dye | | | |
| of the same leaven | | | |
| of two minds | | | |
| oh dark thirty | | | |
| oh my goodness | | | |
| oh my gosh | | | |
| oil and water | | | |
| oil trash | | | |
| old boy network | | | |
| old enough to vote | | | |
| old-fashioned look | | | |
| old fogey | | | |
| old hat | | | |
| old money | | | |
| old saw | | | |
| old song | | | |
| old stick | | | |
| oll korrect | | | |
| on acid | | | |
| on a dime | | | |
| on a full stomach | | | |
| on a lark | | | |
| on a losing wicket | | | |
| on-and-off | | | |
| on another note | | | |
| on a regular basis | | | |
| on a silver platter | | | |
| on a tear | | | |
| on a whim | | | |
| on blast | | | |
| on-brand | | | |
| once and for all | | | |
| once in a blue moon | | | |
| once in a while | | | |
| on cloud nine | | | |
| on course | | | |
| one after another | | | |
| one and all | | | |
| one and only | | | |
| one at a time | | | |
| one brick short of a full load | | | |
| one could hear a pin drop | | | |
| one fell swoop | | | |
| one for the road | | | |
| one-horse race | | | |
| one hundred and one | | | |
| one man and his dog | | | |
| one moment | | | |
| on end | | | |
| one-note | | | |
| one of His Majesty's bad bargains | | | |
| one-percenter | | | |
| one's bark is worse than one's bite | | | |
| one's cake is dough | | | |
| onesey-twosey | | | |
| one's heart out | | | |
| one side | | | |
| one's jig is up | | | |
| one small step for man, one giant leap for mankind | | | |
| one's ship comes in | | | |
| one-star | | | |
| one step at a time | | | |
| one's word is law | | | |
| one thing led to another | | | |
| one too many | | | |
| one-trick pony | | | |
| one-way ticket to Palookaville | | | |