| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 1481 | O' | Irish | Descendent of. Irish surnames formed by prefacing O'. | M | |
| 1482 | O'brian | Irish | Surname. | M | |
| 1483 | O'brien | Irish | Surname. | M | |
| 1484 | O'keefe | Irish | Surname. | M | |
| 1485 | O'shay | Irish | Surname. | M | |
| 1486 | O'shea | Irish | Surname. | M | |
| 1487 | Obharnait | Irish | The color of olive. | F | |
| 1488 | Odanodan | Irish | Of the red earth. | M | |
| 1489 | Odell | Irish | Surname. | M | |
| 1490 | Odhran | Irish | Pale green. | M | |
| 1491 | Odran | Irish | Pale green. | M | |
| 1492 | Oengus | Irish | One vigor. | M | |
| 1493 | Ohnicio | Irish | Honor. | F | |
| 1494 | Oilbhe | Irish | Olive. | F | |
| 1495 | Olive | Irish | Olive. | F | |
| 1496 | Oma | Irish | The color of olive. | F | |
| 1497 | Omat | Irish | The color of olive. | F | |
| 1498 | Ona | Irish | One. Also a variant of Una: Lamb; together. | F | |
| 1499 | Onora | Irish | Honor. | F | |
| 1500 | Oona | Irish | Variant of Oonagh: Lamb. Can also be an Irish variant of Latin Una: One. | F | |