| Only occupant of Vostok I | 25 |
| Only one-syllable state | 23 |
| Only one-syllable state name | 28 |
| Only openings for air? | 22 |
| Only Pacific island monarchy | 28 |
| Only part of Egypt in Asia | 26 |
| Only pope from England | 22 |
| Only pres. born in Missouri | 27 |
| Only president born in Hawaii | 29 |
| Only remaining heater? | 22 |
| Only son of Czar Nicholas II | 28 |
| Only South Dakota-born VP | 25 |
| Only state with homonyms | 24 |
| Only tavern in Springfield | 26 |
| Only this, and nothing else | 27 |
| Only three-letter element | 25 |
| Only U.S. pres. to resign | 25 |
| Only Veep from Maryland | 23 |
| Only wants for sex, say | 23 |
| Only Welsh-born Batman | 22 |
| Only what a person can take? | 28 |
| Only with for the money, say | 28 |
| Only woman to outwit Holmes | 27 |
| Only women understand one | 25 |
| Onomastician's study | 24 |
| Onomasticon's contents | 26 |
| Onomatopoeically named legume | 29 |
| Onomatopoetic water sound | 25 |
| Onset of enthusiasm, perhaps | 28 |
| Onslaught of cold weather | 25 |
| Onstage thought balloon | 23 |
| Onstage thought bubble | 22 |
| Ontario Canadian Forces Base | 28 |
| Ontario Freeway, colloquially | 29 |
| Ontario heritage river | 22 |
| Ontario or Supérieur | 23 |
| Ontario Park, Fort ______ | 26 |
| Ontario politician Bob ___ | 26 |
| Ontario Premier George | 22 |
| Ontario Premier Leslie | 22 |
| Ontario Premier, formerly | 25 |
| Ontario river to James Bay | 26 |
| Ontario's 21st premier | 26 |
| Ontario's 23rd Premier | 26 |
| Ontario's Fort ___ | 22 |
| Ontario's Little _____ | 26 |
| Ontario's Michelle ______ | 29 |
| Ontario's neighbor | 22 |
| Ontario's official bird | 27 |
| Ontario's Old ___ William | 29 |
| Ontario's Premier David | 27 |
| Ontario's Premier Ernie | 27 |
| Ontario's provincial bird | 29 |
| Ontario's Red _____ | 23 |
| Ontario's Sault ___ Marie | 29 |
| Ontario's Shania ____ | 25 |
| Ontario's __ Tower | 22 |
| Ontario's ___ Canal | 23 |
| Ontario's ___ Canals | 24 |
| Ontario's ___ Locks | 23 |
| Ontario's ____ Hill | 23 |
| Ontario's ____ locks | 24 |
| Ontario's ____Centre | 24 |
| Ontario's _____ Carl Roy | 29 |
| Ontario's _____ Dick | 24 |
| Ontario's _____ Hill | 24 |
| Ontario's _____ Lake | 24 |
| Ontario's _____Bernardo | 27 |
| Ontario's _____Demeter | 26 |
| Ontario's ______ Canal | 27 |
| Ontario's ______canal | 25 |
| Ontario-born hockey legend | 26 |
| Ontario-Quebec border river | 27 |
| OntarioÂ’s Sault ___ Marie | 29 |
| Onto, with "of" | 25 |
| Ontologist's concern | 24 |
| Ontologists' concerns | 25 |
| Onyx hit about impact? | 22 |
| Oologist's subject | 22 |
| Oologists' studies | 22 |
| Oom Paul Kruger, for one | 24 |
| Oom-pah-pah instrument | 22 |
| Oom-___ (brass band sounds) | 27 |
| Oompa-Loompas' boss | 23 |
| Oompa-Loompas' creator | 26 |
| Oompah band instrument | 22 |
| Oop's main squeeze | 22 |
| Oops, she had a career | 22 |
| Ooze through a crevice | 22 |
| Ooze through the cracks | 23 |
| Ooze, as through cracks | 23 |
| Oozing monster of 1958 | 22 |
| Oozy horror film menace | 23 |
| Op ___ (footnote abbr.) | 23 |
| Op ___ (in the work cited) | 26 |
| Op ____: in the work cited | 26 |
| Op. -- (abbr. in a footnote) | 28 |
| Op. --. (where mentioned) | 25 |
| Op. __ (footnote notation) | 26 |
| Op. __: footnote abbr. | 22 |