| City | Administrative Division | Country | |
| Oslo | Oslo | Norway | |
| Osnabrück | Lower Saxony | Germany | |
| Osoyoos | British Columbia | Canada | |
| Oss | Noord Brabant | The Netherlands | |
| Oswego | New York | United States | |
| Otaru | Hokkaido | Japan | |
| Ottawa | Ontario | Canada | |
| Otterburn | England | United Kingdom | |
| Ottumwa | Iowa | United States | |
| Ouary | Colorado | United States | |
| Overland Park | Kansas | United States | |
| Ovid | New York | United States | |
| Owen Sound | Ontario | Canada | |
| Owings | Maryland | United States | |
| Owings Mills | Maryland | United States | |
| Oxford | England | United Kingdom | |
| Oxford | Mississippi | United States | |
| Oxford | Ohio | United States | |
| Oxfordshire | England | United Kingdom | |
| Oxnard | California | United States | |
| Oxon Hill | Maryland | United States | |
| Oxton | England | United Kingdom | |
| Oyster Bay | New York | United States | |
