| on fire | | | |
| on good terms | | | |
| on hand | | | |
| on hold | | | |
| on ice | | | |
| on in years | | | |
| on its merits | | | |
| only game in town | | | |
| only time will tell | | | |
| on one's bill | | | |
| on one's deathbed | | | |
| on one's feet | | | |
| on one's high horse | | | |
| on one's last legs | | | |
| on one's lonesome | | | |
| on one's mind | | | |
| on one's tod | | | |
| on one's toes | | | |
| on one's watch | | | |
| on opposite sides of the barricades | | | |
| on pins and needles | | | |
| on purpose | | | |
| on-ramp | | | |
| on second thought | | | |
| on sight | | | |
| on solid ground | | | |
| on someone's account | | | |
| on someone's dime | | | |
| on someone's doorstep | | | |
| on someone's hands | | | |
| on someone's mind | | | |
| on someone's radar | | | |
| on steroids | | | |
| on strike | | | |
| on stun | | | |
| on sufferance | | | |
| on talking terms | | | |
| on tenterhooks | | | |
| on the anvil | | | |
| on the back burner | | | |
| on the back foot | | | |
| on the back of | | | |
| on the ball | | | |
| on the beach | | | |
| on the boob | | | |
| on the bounce | | | |
| on the brain | | | |
| on the breadline | | | |
| on the bubble | | | |
| on the button | | | |
| on the canvas | | | |
| on the cards | | | |
| on the clock | | | |
| on the cuff | | | |
| on the cusp | | | |
| on the cutting room floor | | | |
| on the debit side | | | |
| on the dot | | | |
| on the double | | | |
| on the down-low | | | |
| on the drum | | | |
| on the dry | | | |
| on the edge | | | |
| on the edge of one's seat | | | |
| on the face of | | | |
| on the face of it | | | |
| on the flip side | | | |
| on the fly | | | |
| on the fritz | | | |
| on the front burner | | | |
| on the front foot | | | |
| on the game | | | |
| on the go | | | |
| on the gripping hand | | | |
| on the heels of | | | |
| on the high rope | | | |
| on the high ropes | | | |
| on the hoof | | | |
| on the hook | | | |
| on the horn | | | |
| on the horns of a dilemma | | | |
| on the house | | | |
| on the hush | | | |
| on the ladder | | | |
| on the lam | | | |
| on the level | | | |
| on the lines of | | | |
| on the loose | | | |
| on the mark | | | |
| on the mat | | | |
| on the money | | | |
| on the move | | | |
| on the nose | | | |
| on the one hand | | | |
| on the other hand | | | |
| on the other side of | | | |
| on the outs | | | |
| on the outside, looking in | | | |
| on the pill | | | |
| on the plus side | | | |