Cities and Communities in Southern Ontario
(check also Northern Ontario)
| Oak Lake | Orangeville | Ospringe |
| Oak Orchard | Orillia | Ossossane Beach |
| Oak Ridges | Orleans | Ostrander |
| Oakdale | Ormsby | Ostryhon Corners |
| Oakland | Oro Beach | Ottawa |
| Oakville | Oro Lea Beach | Otter Lake |
| Oakwood | Oro Station | Otterville |
| Odessa | Orono | Ouellette |
| Ohsweken | Orr Lake | Oustic Outlet (Leeds and Grenville) |
| Oil City | Orrville | Outlet (Prince Edward) |
| Oil Springs | Orton | Owen Sound |
| Oldcastle | Orwell | Oxenden |
| Oliphant | Osaca | Oxford Centre |
| Oliver | Osborne Corners | Oxford Mills |
| Omemee | Osceola | Oxley |
| Ompah | Osgoode | Oxtongue Lake |
| Onondaga | Oshawa | |
| Onyotaaka | Osnabruck Centre |