O
| Class | Country | Type | Launch years | Displacement (tons) | Number |
| O and P | UK | destroyer | 1939-42 | 1,610 | 16 |
| Odin | Norway | destroyer | 1939 | 632 | 3 |
| Omaha | US | light cruiser | 1920-24 | 7,050 | 10 |
| OrzeĊ | Poland | submarine | 1,100 | 2 | |
| Otori | Japan | torpedoboat | 1935-37 | 840 | 8 |
Read more about this topic: List Of World War II Ship Classes