- O'Brien, Dermod PRHA, HRA
- O'Brien, Fr. John
- O'Brien, Geraldine
- O'Brien, Kieran Doyle
- Ó'Colmáin, Séamus
- O'Conchubhair, P
- O'Connor, John
- O'Connor, Sean
- O'Donnell, Deirdre
- O'Halloran, James
- O'Hara, Helen
- O'Hare, M.
- O'Kelly, Carmel
- O'Malley, Tony HRHA
- O'Neill, Ken
- O'Neill, Mark
- O'Ryan, Fergus RHA
- O'Sullivan, Sean RHA
- O'Sullivan, Seán RHA
- Olsson, Albert Julius
- Orlandi, Barbara
- Orpen, Sir William RA,RHA, RWS
