| Name |
Pronunciation |
Meaning |
English Spelling |
Gender |
| Ócán, Ógán | (OG-awn) | youth, lad | Hogan | M |
| Odar, Odhar | (OHR) | dark, grey-brown | | M |
| Odarnat, Odharnait, Órnait | (OR-nit) | dark, grey-brown | Orna, Orna | F |
| Odrán Odhrán, Órán | (oh-RAWN) | dark, grey-brown | Oren, Horan | M |
| Óengus, Áengus, Aonghus | (EEN-us) | | Enos | M |
| Óenu, Aona | (EEN-a) | pet form of Óengus | | M |
| Óenucán, Aonagán | (EN-eg-awn) | | Hannigan | M |
| Oilléoc, Oilleóg | (IL-og) | | | M |
| Oissíne, Oisséne, Oisín | (ush-EEN) | dim. of oss, "a deer" legendary name | | M |
| Olcán | (ul-KAWN) | a wolf | | M |
| Ólchobar, Ólchobhar | (OL-kur) | | | M |
| Olibhéar | (OL-iv-er) | borrowing of Oliver | | M |
| Onchú | (UN-koo) | fierce hound | | M |
| Onóra | (o-NOR-a) | borrowing of Anglo-Saxon "Honora" reputation, beauty | Nora, Noreen, Honora | F |
| Órlaith, Órla | (OR-la) | princess | Orla | F |
| Órnat, Órnait | (OR-nit) | | | F |
| Orthanach | (UR-han-ach) | powerful in charms | | M |
| Oscar, Osgar | (US-kar) | deer-lover | Oscar | M |
| Oschú | (US-koo) | deer-hound | | M |
| Ossán, Osán | (us-AWN) | dim. of oss, a deer | | M |
| Ossnat, Ossnait | (US-nit) | deer, fem. form of Ossán | | F |