| Enumerated Area | pre1851 | 1851 | 1861 | 1871 | 1881 | 1891 | 1901 | 1911 |
| Oakland Twp | BRA | BRA | BRA | BRA | OXF | OXF | BRA | |
| Oakland Village | BRA | |||||||
| Oakley Twp | MUS | MUS | ONT | ONT | MUS | |||
| Oakville Town | HTN | HTN | HTN | HTN | HTN | HTN | ||
| Obabikon Lake | TBRR | |||||||
| O'Brien Siding | ALG | |||||||
| Obrig Twp | NIP | |||||||
| O'Connor | TBRR | |||||||
| Odessa Village | LnA | |||||||
| Ogden | ALG | |||||||
| Ohsweken Village | BRA | |||||||
| Oil City | LAM | |||||||
| Oil Springs Village | LAM | LAM | LAM | LAM | LAM | |||
| Ojibwa Indian Settlement Twp | MID | |||||||
| Ojibwa Indians | ALG | |||||||
| Old Mille Roches | STO | |||||||
| Olden Twp | FRO | LnA | LnA | LnA | LnA | FRO | ||
| Oliver Twp | ALG | TBRR | ||||||
| Olrig | NIP | |||||||
| Olser Twp | NIP | |||||||
| Omemee Village | VIC | VIC | VIC | VIC | ||||
| Ompah | FRO | |||||||
| Oneida Indian Reserve | HLD | MID | ||||||
| Oneida Twp | HLD | HLD | HLD | HLD | HLD | HLD | HLD | |
| Onondago Twp | BRA | BRA | BRA | BRA | BRA | BRA | BRA | BRA |
| Onondago Village | BRA | |||||||
| Ops Twp | VIC | VIC | VIC | VIC | VIC | VIC | VIC | |
| Orangeville Town | WGT | WGT | WGT | WGT | DUF | |||
| Orford Maranian Reserve | KEN | |||||||
| Orford Twp | KEN | KEN | KEN | KEN | ELG | ELG | KEN | |
| Orillia Town | SIM | SIM | SIM | SIM | SIM | |||
| Orillia Twp | SIM | SIM | SIM | SIM | SIM | SIM | SIM | |
| Orland Village | NUL | |||||||
| Oro Station | SIM | SIM | ||||||
| Oro Twp | SIM | SIM | SIM | SIM | SIM | SIM | SIM | |
| Orrville Village | PSD | |||||||
| Orton Village | DUF | |||||||
| Orwell Village | ELG | |||||||
| Osgoode Twp | RUS | MS | RUS | RUS | RUS | RUS | RUS | |
| Osgoode Twp | CAR | CAR | ||||||
| Oshawa Town | ONT | ONT | ONT | ONT | ONT | |||
| Oshawa Village | ONT | |||||||
| Osnabruck Twp | STO | STO | STO | STO | STO | STO | STO | |
| Oso Twp | FRO | LnA | LnA | LnA | LnA | FRO | ||
| Osprey Twp | GRY | GRY | GRY | GRY | GRY | GRY | GRY | |
| Ospring | WGT | |||||||
| Otonabee Twp | PET | PET | PET | PET | PET | PET | PET | PET |
| Ottawa City | CAR | CAR | CAR | CAR | CAR | CAR | ||
| Ottawa East Village | CAR | CAR | CAR | |||||
| Ottawa South | CAR | |||||||
| Otter | ALG | |||||||
| Otter Station | ALG | |||||||
| Otter Tail Twp | ALG | |||||||
| Otterville Village | OXF | |||||||
| Owen Sound Town | GRY | GRY | GRY | GRY | GRY | GRY | ||
| Oxdorf | ALG | |||||||
| Oxenden Village | GRY | GRY | GRY | |||||
| Oxford (Oxford-on-the-Rideau) Twp | GVL | GVL | GVL | GVL | GVL | GVL | GVL | GVL |
| Oxford Centre | OXF | |||||||
| Oxford East Twp | OXF | OXF | OXF | OXF | OXF | OXF | OXF | |
| Oxford Mills Village | GVL | |||||||
| Oxford North Twp | OXF | OXF | OXF | OXF | OXF | OXF | ||
| Oxford West Twp | OXF | OXF | OXF | OXF | OXF | OXF | OXF |