| Odhran | Man | Irish Names |
| Oisin | Man | Irish Names |
| Oistin | Man | Irish Names |
| Oona | Woman | Irish Names |
| Oonagh | Woman | Irish Names |
| Oran | Unisex | Irish Names |
| Orla | Woman | Irish Names |
| Orlaith | Woman | Irish Names |
| Ormanda | Woman | Irish Names |
| Ormond | Man | Irish Names |
| Orrin | Man | Irish Names |
| Oscar | Man | Irish Names |
| Ossian | Man | Irish Names |
| Owen | Man | Irish Names |
| Ownah | Woman | Irish Names |