| Ontario is on it | 16 |
| Ontario Indian | 14 |
| Ontario Hockey League team from Erie | 36 |
| Ontario heritage river | 22 |
| Ontario Freeway, colloquially | 29 |
| Ontario Fort | 12 |
| Ontario Forces Base | 19 |
| Ontario feeder | 14 |
| Ontario county | 14 |
| Ontario community or the former Taiwan | 38 |
| Ontario community or British namesake | 37 |
| Ontario community or English namesake | 38 |
| Ontario community | 17 |
| Ontario city just west of Buffalo | 33 |
| Ontario city | 12 |
| Ontario cape or former Tory leader Stanfield | 44 |
| Ontario Canadian Forces Base | 28 |
| Ontario airbase | 15 |
| Ontario Ag school | 17 |
| Ontario address | 15 |
| Ontario for one | 16 |
| Ontario export | 15 |
| Ontario community or Polish namesake | 37 |
| Ontarians' national anthem | 30 |
| Ont., for one | 13 |
| Ont.'s neighbor | 19 |
| Ont.'s Capital | 18 |
| Ont. time zone | 14 |
| Ont. summer time | 16 |
| Ont. site of a War of 1812 battle | 33 |
| Ont. police force | 17 |
| Ont. or Sask. | 13 |
| Ont. or Que. | 12 |
| Ont. neighbor | 13 |
| Ont. military college | 21 |
| Ont. channel | 12 |
| Ont. appliance maker | 20 |
| OnStar provider | 15 |
| Onstage, say | 12 |
| Onstage thought bubble | 22 |
| Onstage thought balloon | 23 |
| Onstage surprise | 16 |
| Onstage strutters | 17 |
| Onstage phone, e.g. | 19 |
| Onstage head costume | 20 |
| Onstage ham | 11 |
| Onstage gesture | 15 |
| Onstage extra during an aria, for short | 39 |
| Onstage digression | 18 |
| Onstage boo-boo | 15 |
| Onstage blunder | 15 |
| Onslaughts | 10 |
| Onslaught of crepe orders at a deli? | 36 |
| Onslaught of cold weather | 25 |
| Onslaught | 9 |
| Onsite journalist | 17 |
| Onshore | 7 |
| Onsets | 6 |
| Onset of wakefulness | 20 |
| Onset of phobia? | 16 |
| Onset of melting | 16 |
| Onset of labor? | 15 |
| Onset of evil? | 14 |
| Onset of enthusiasm, perhaps | 28 |
| Onset of boredom? | 17 |
| Onset of a lie? | 15 |
| Onset of | 8 |
| Onset | 5 |
| Ons' opposites | 18 |
| Ons and ____ | 12 |
| Onrush | 6 |
| Onomatopoetic water sound | 25 |
| Onomatopoetic sound in KRS-One's "Sound of da Police" | 67 |
| Onomatopoeically named legume | 29 |
| Onomatopoeic word | 17 |
| Onomatopoeic sound | 18 |
| Onomatopoeic Sinatra album title | 32 |
| Onomatopoeic game on "The Price Is Right" | 51 |
| Onomatopoeic drumbeat | 21 |
| Onomatopoeic dance | 18 |
| Onomatopoeia | 12 |
| Onomastics study | 16 |
| Onomasticon's contents | 26 |
| Onomastician's study | 24 |
| Ono/Lennon album | 16 |
| Ono's love | 14 |
| Ono of JAPAN | 12 |
| Ono | 3 |
| Only: Ger. | 10 |
| Only: Fr. | 9 |
| Only, in Ulm | 12 |
| Only, in Oldenburg | 18 |
| Only, in Offenbach | 18 |
| Only, in Bremen | 15 |
| Only, in Bonn | 13 |
| Only's partner | 18 |
| Only's companion | 20 |
| Only ___ a customer | 19 |
| Only you | 8 |
| Only X-rated film to win the Best Picture Oscar | 47 |