| Ollie | Man | English Names |
| Onyx | Unisex | English Names |
| Orchid | Unisex | English Names |
| Orde | Man | English Names |
| Ordell | Unisex | English Names |
| Ormand | Man | English Names |
| Orrick | Man | English Names |
| Orsen | Man | English Names |
| Orsin | Man | English Names |
| Orson | Man | English Names |
| Orville | Man | English Names |
| Osbert | Man | English Names |
| Osborn | Man | English Names |
| Osbourne | Man | English Names |
| Osgood | Man | English Names |
| Osmond | Man | English Names |
| Osmund | Man | English Names |
| Osric | Man | English Names |
| Ossie | Man | English Names |
| Ostara | Woman | English Names |