| Nyassa |
|
| Obock |
|
| Ohio (OH) |
|
| Oil Rivers Protectorate |
|
| Oklahoma (OK) |
|
| Oldenburg |
|
| Oltre Biuba |
|
| Oman |
|
| Oman, Dhufar |
|
| Ontario |
|
| Orange Free State |
|
| Orange River Colony |
|
| Orchha |
|
| Oregon (OR) |
|
| Pahang |
|
| Pahang, Japanese Occupation |
|
| Pakistan |
|
| Pakistan- Revenue Stamps |
|
| Palau |
|
| Palestine, British Mandate |
|