| oll korrect | | | |
| on account of | | | |
| on acid | | | |
| on a day-to-day basis | | | |
| on a dime | | | |
| on a first-name basis | | | |
| on a full stomach | | | |
| on a kick | | | |
| on a lark | | | |
| on all fours | | | |
| on a losing wicket | | | |
| on and off | | | |
| on-and-off | | | |
| on an irregular basis | | | |
| on another note | | | |
| on a plate | | | |
| on a regular basis | | | |
| on a shoestring | | | |
| on a silver platter | | | |
| on a sixpence | | | |
| on a tear | | | |
| on average | | | |
| on a whim | | | |
| on bad terms | | | |
| on blast | | | |
| on board | | | |
| on-brand | | | |
| once again | | | |
| once and for all | | | |
| once bitten, twice shy | | | |
| once in a blue moon | | | |
| once in a purple moon | | | |
| once in a while | | | |
| once or twice | | | |
| on cloud nine | | | |
| on cloud seven | | | |
| on course | | | |
| on demand | | | |
| one after another | | | |
| one age with | | | |
| one and all | | | |
| one-and-done | | | |
| one and only | | | |
| one and the same | | | |
| one at a time | | | |
| one big happy family | | | |
| one brick short of a full load | | | |
| one could do it in one's sleep | | | |
| one could hear a pin drop | | | |
| on edge | | | |
| one fell swoop | | | |
| one flesh | | | |
| one for the road | | | |
| one-hit wonder | | | |
| one-horse race | | | |
| one-horse town | | | |
| one hundred and one | | | |
| one in the eye for | | | |
| one man and his dog | | | |
| one-man band | | | |
| one moment | | | |
| one more again | | | |
| on end | | | |
| one-night stand | | | |
| one-note | | | |
| one-off | | | |
| one of His Majesty's bad bargains | | | |
| one of those things | | | |
| one-percenter | | | |
| one's back is up | | | |
| one's bark is worse than one's bite | | | |
| one's blood is up | | | |
| one's cake is dough | | | |
| one's days are numbered | | | |
| onesey-twosey | | | |
| one's heart in one's mouth | | | |
| one's heart out | | | |
| one's hour | | | |
| one side | | | |
| onesie-twosie | | | |
| one's jig is up | | | |
| one's lips are sealed | | | |
| one small step for man, one giant leap for mankind | | | |
| one's name is mud | | | |
| one's ship comes in | | | |
| one's socks off | | | |
| one-star | | | |
| one step ahead | | | |
| one step at a time | | | |
| one step forward, two steps back | | | |
| one's word is law | | | |
| on eternal patrol | | | |
| one thing led to another | | | |
| one thing or another | | | |
| one too many | | | |
| one-track mind | | | |
| one-trick pony | | | |
| one up | | | |
| one-way ticket to Palookaville | | | |
| on file | | | |