| Name | Gender | Pronounced | Usage | Popularity |
|---|---|---|---|---|
| Oonagh | OO-na | Irish | ||
| Oran | O-ran | Irish | ||
| Orla (1) | OR-la | Irish | ||
| Orlagh | - | Irish | ||
| Orna (1) | - | Irish | ||
| Ornat | - | Irish | ||
| Orrin | - | Irish | ||
| Oscar | AHS-kər (English) | English, Irish, Portuguese (Brazilian), Italian, Swedish, Norwegian, Danish, Dutch, French, Irish My | ||
| Osheen | - | Irish | ||
| Owen (2) | - | Irish | ||
| PÁDraic | PAW-drik | Irish | ||
| PÁDraig | PAW-drig, PAW-rig | Irish | ||
| PÁDraigÍN | PAW-dri-geen | Irish | ||
| PÓL | POL | Irish | ||
| Paddy | - | Irish |