| Registered Name | Owner / Club | Status |
|---|---|---|
| Oops I Dun It Again | Patricia Warren Donegal Gaeltacht | Approved |
| Oos Rosie | Melissa McVeigh Border Counties | Approved |
| Oppenheimer Blue (IRE) | Aisling McGillicuddy Tara Hill | Approved |
| Orangeville | Claire Maher Killcarrick | Approved |
| Orbis View | Aine Doyle Rathfarnham | Approved |
| Orchard Athena D'Or | Kevin McGirr Lakelands | Approved |
| Orchard Hill Star | Kevin McCarron Drumcoura | Approved |
| Orchard Moonlight | Susan Wilson Tara Hill | Approved |
| Orchard Sundown (IRE) | Liam Kelly Devils Bit | Approved |
| Order of the Phoenix | Catriona Mills Broadmeadow | Approved |
| Oreo (Cobid) | Linda Byrne Cheval | Approved |
| Oreo Cookie | Esther O' Kane Donegal Gaeltacht | Approved |
| Oreo Lad | Lynsey Rogers Headfort | Approved |
| Oreo Magic | Paula Campbell Barnacarroll | Approved |
| Oreo Sally Lady | Shelley Golden Rathfarnham | Approved |