| On an airplane | 14 |
| On an aimless course | 20 |
| On an ad -- basis | 17 |
| On all ___ (crawling, say) | 26 |
| On all sides | 12 |
| On all counts | 13 |
| On all --- (crawling) | 21 |
| On all -- (crawling) | 20 |
| On alert | 8 |
| On Alcatraz, e.g. | 17 |
| On air trucker? | 15 |
| On air icon | 11 |
| On again, in a way | 18 |
| On again, as a lantern | 22 |
| On again, as a candle | 21 |
| On again | 8 |
| ON address, or Disney duck | 26 |
| ON address, or actress Lohan | 28 |
| On account of (with ''to'') | 43 |
| On account of | 13 |
| On account (of) | 15 |
| On a ___-to-know basis | 22 |
| On a ___ (raging) | 17 |
| On a ___ (carousing) | 20 |
| On a ___ | 8 |
| On a __-to-know basis | 21 |
| On a yacht, briefly | 19 |
| On a winning streak | 19 |
| On a wing ___ prayer | 20 |
| On a windjammer | 15 |
| On a wayward path | 17 |
| On a warpath | 12 |
| On a voyage on the ocean | 24 |
| On a voyage | 11 |
| On a vessel's left | 22 |
| On a vacation | 13 |
| On a train | 10 |
| On a tilt | 9 |
| On a tight schedule | 19 |
| On a tether | 11 |
| On a tear | 9 |
| On a tandem, e.g. | 17 |
| On a streak? | 12 |
| On a streak | 11 |
| On a steamer, say | 17 |
| On a spree | 10 |
| On a spit, say | 14 |
| On a Spanish ship | 17 |
| On a sortie | 11 |
| On a sofa, say | 14 |
| On a sofa | 9 |
| On a smaller scale | 18 |
| On a small scale | 16 |
| On a slow roller: "A Midsummer Night's Dream" | 59 |
| On a slow boat to China, e.g. | 29 |
| On a slow boat to China | 23 |
| On a slow boat to | 17 |
| On a sloop's sheltered side | 31 |
| On a slant: Abbr. | 17 |
| On a slant | 10 |
| On a single occasion | 20 |
| On a single number, in roulette | 31 |
| On a shuttle, maybe | 19 |
| On a shortage of know-how? | 26 |
| On a short leash | 16 |
| On a shoestring | 15 |
| On a ship | 9 |
| On a seesaw | 11 |
| On a schooner, say | 18 |
| On a scale of ___ 10 | 20 |
| On a scale of one ___ | 21 |
| On a scale of one __ | 20 |
| On a scale of one -- | 20 |
| On a scale of 1 to 10, what one amp in "This Is Spinal Tap" goes to | 77 |
| On a scale from one -- | 22 |
| On a safari | 11 |
| On a run | 8 |
| On a roll | 9 |
| On a recurring basis | 20 |
| On a rampage | 12 |
| On a queue | 10 |
| On a quest for a modern musical style? | 38 |
| On a quest for a certain spicy food? | 36 |
| On a proportional basis | 23 |
| On a plane? | 11 |
| On a plane, train, or bus | 25 |
| On a plane, train or bus | 24 |
| On a plane | 10 |
| On a pinnacle | 13 |
| On a pension: Abbr. | 19 |
| On a pension, briefly | 21 |
| On a pension (Abbr.) | 20 |
| On a pens. | 10 |
| On a pedestal | 13 |
| On a PC, it does nothing by itself | 34 |
| On a par: Prefix | 16 |
| On a par: Fr. | 13 |
| On a par, in Paris | 18 |
| On a par, in Orléans | 23 |
| On a par with, in France | 24 |