| 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 |
| Only World Series perfect game pitcher | 38 |
| Only work that Michelangelo signed | 34 |
| Only work of art that Michelangelo signed | 41 |
| Only word spoken in Mel Brooks's "Silent Movie" | 61 |
| Only word heard in a 1958 song of the same name | 47 |
| Only word containing three consecutive pairs of double letters | 62 |
| Only women understand one | 25 |
| Only woman to win the top prize on "The $64,000 Question" | 67 |
| Only woman to outwit Holmes | 27 |
| Only with for the money, say | 28 |
| Only winner of People's Sexiest Man Alive who's no longer alive | 71 |
| Only winner of eight consecutive Norris Trophies | 48 |
| Only Wimbledon winner to wear a wig | 35 |
| Only Wimbledon champion to wear a wig | 37 |
| Only when his funny films failed | 32 |
| Only what a person can take? | 28 |
| Only Welsh-born Batman | 22 |
| Only wants for sex, say | 23 |
| Only vegetable never sold frozen, canned, processed, or cooked | 62 |
| Only Veep from Maryland | 23 |
| Only valid three-letter Scrabble word that doesn't contain Y or any consonant | 81 |
| ONLY US, HANS!: 1983 | 20 |
| Only US swimmer to compete in five Olympics | 43 |
| Only US state with two official languages | 41 |
| Only US state with a diamond mine | 33 |
| Only US state that can be typed on one row of the QWERTY keyboard | 65 |
| Only US president with a star on the Hollywood Walk of Fame | 59 |
| Only US president to receive both a Purple Heart and a Pulitzer Prize | 69 |
| Only ungulate that mates in the seated position | 47 |
| Only U.S. vice president born in Maryland | 41 |
| Only U.S. state without a rectangular flag | 42 |
| Only U.S. state with a pennant-shaped flag | 42 |
| Only U.S. state whose flag isn't rectangular | 48 |