- O'Brien, Baroness Annie
- O'Brien, Christopher Michael
- O'Brien, His Honour Judge Daniel Joseph
- O'Brien, Dermod
- O'Brien, Hon. Donough Edward Foster
- O'Brien, Hon. Donough
- O'Brien, Edmond Lyons
- O'Brien, Hon. Edward Donough
- O'Brien, Colonel Eoghan
- O'Brien, George
- O'Brien, Capt. Hon. Henry Barnaby
- O'Brien, John Robert
- O'Brien, Kendal Edmund
- O'Brien, Hon. Lucius Murrough
- O'Brien, Lt.-Col. Hon. Murrough
- O'Brien, Hon. Phadrig Luciue Ambrose
- O'Brien, Sir Timothy Carew
- O'Brien, Torlogh
- O'Brien, Alderman William
- O'Brien, William
- O'Brien, Hon. William Henry Robert Ernest Turlough
- O'Byrne, Count John Edward Gerald O'Byrne
- O'Callaghan, Daniel
- O'Callaghan, Mrs. Kate
- O'Callaghan-Westropp, Col. George
- O'Carroll, Joseph Francis
- O'Ceallaigh, Sean T.
- O'Connell, Daniel Charles
- O'Connell, Rev. Frederic Wm.
- O'Connell, Major James Ross
- O'Connell, John Joseph
- O'Connell, Sir John Robert
- O'Connell, Capt. Sir Maurice James Arthur
- O'Connell, Sir Peter Reilly
- O'Connell, Lady Mary Pauline
- O'Connell, Thomas Joseph
- O'Connor, Arthur John
- O'Connor, Right Hon. Charles Andrew
- O'Connor, George Richard
- O'Connor, Right Hon. James
- O'Connor, Lieut.-Col. Patrick Fenelon
- O'Connor, Thomas Power
- O'Conor, Charles Hugh
- O'Conor, Edmond
- O'Conor, Lady Minna Margaret
- O'Conor Don, The, Owen Phelim O'Conor
- O'Doherty, Eileen
- O'Doherty, James Edward
- O'Doherty, Joseph
- O'Doherty, Philip
- O'Doherty, Most Rev. Thomas
- O'Donnell, Charles James
- O'Donnell, Elliot
- O'Donnell, John
- O'Donnell, Rev. Michael J.
- O'Donnell, Thomas (M.P. West Kerry)
- O'Donnell, Thomas (T.D. Sligo–Mayo East)
- O'Donnell, Lieut.-Gen. Sir Thomas Joseph
- O'Donoghoe, Sir James
- O'Donoghue, Geoffrey (The O'Donoghue of the Glens)
- O'Donoghue, Tadhg
- O'Donoghue, Thomas
- O'Donovan, Fred
- O'Donovan, The, Morgan William O'Donovan
- O'Dowd, John
- O'Duffy, General Owen
- O'Dwyer, Sir Michael Francis
- O'Dwyer, Robert
- O'Farrell, Charles Richard
- O'Farrell, Sir Edward
- O'Farrell, Surgeon-General Thomas
- O'Farrell, Thomas T.
- O'Farrelly, Miss Agnes
- O'Ferrall, John Forbes
- O'Ferrall, Dominic More-
- O'Flaherty, Samuel
- O'Flanagan, Reverend Michael
- Ogilby, Lt.-Col. Robert James Leslie
- O'Gorman, Col. Nicholas Purcell O'Gorman (The O'Gorman)
- O'Grady, Guillamore
- O'Grady, Major Hugh Hamon Massy
- O'Grady, William De Rienzi O'Grady (The O'Grady)
- O'Hara, Arthur Cooper
- O'Hara, Charles Kean
- O'Hara, Lt.-Col. Erril Robert
- O'Hara, Right Rev. Henry Stewart
- O'Hara, Colonel James
- O'Hara, Major Patrick Henry Augustus
- O'Higgins, Brian
- O'Keeffe, Major-Gen. Sir Manus William
- O'Keeffe, Patrick
- O'Keeffe, Stephen Charles Martin Lanigan-
- O'Kelly, De Gallagh et Tycooly, Count Gerald Edward
- O'Kelly, Capt. Henry Kane
- O'Kelly, John Thomas
- O'Kelly, Sean Joseph (Sceilig)
- O'Kelly, Rev. Thomas
- O'Kennedy, Very Rev. Canon Richard
- O'Leary, Daniel Henry
- O'Loghlen, Sir Michael
- O'Loingsigh, Fionan
- O'Mahony, Sean
- O'Mahony, Pierce Charles de Lacy O'Mahony (The O'Mahony)
- O'Maille, Prof. Tomas
- O'Maille, Padraic
- O'Malley, Ellen
- O'Malley-Keyes, Major Middleton
- O'Mara, James
- O'Mara, Joseph
- O'Mara, Alderman Stephen Mary
- O'Meagher, Col. John Kevin
- O h-Aodha, Padraig
- O'Neill, Baron
- O'Neill, Major Edward Michael
- O'Neill, Rev. George
- O'Neill, Rev. John
- O'Neill, Rev. Patrick
- O'Neill, Right Hon. Laurence
- O'Neill, Maire
- O'Neill, Patrick
- O'Neill, Rt. Hon. R. W. Hugh
- O'Neill, Colonel William Henry Slingsby
- O'Neill-Power, Captain John Joseph
- O'Nolan, Rev. Gerald
- O'Rahilly, Alfred
- O'Rahilly, Thomas F.
- Oranmore and Browne, Baron
- O'Reilly, Captain Ferdinand Hugh Luke de la Pere
- O'Reilly, Lt.-Col. Charles Myles
- O'Reilly, William Joseph
- Orme, Christopher Guy
- Ormonde, Marquess of
- Ormsby, Anthony
- Ormsby, Colonel Gilbert John Anthony
- Ormsby, Lieut.-Col. Thomas
- Ormsby, Sir Lambert Hepenstal
- Ormsby-Gore, Hon. William George Arthur
- O'Rorke, Charles Trench
- Orpen, Goddard Henry
- Orpen, R. Caulfeild
- Orpen, Major Sir William
- Orr, Very Rev. John
- Orr, Major Michael Harrison
- Orr, Thomas
- Orr, William M'Fadden
- Osborne, Rev. James Denham
- Osborne, His Honor Judge Robert Ernest
- O'Shannon, Cathal
- O'Shaughnessy, M. M.
- O'Shaughnessy, Patrick Joseph
- O'Shaughnessy, Richard
- O'Shaughnessy, The Right Hon. Thomas Lopdell
- O'Shaunessy, George Francis
- O'Shea, Sir Henry
- O'Shee, Col. George Iver Patrick Poer-
- O'Shee, James John
- O'Shee, Lieut.-Col. Richard Alfred Poer-
- Ossory, Bishop of
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- Ossory, Earl of
- O'Sullivan, Eugene
- O'Sullivan, Gerald
- O'Sullivan, Professor John M.
- O'Sullivan, Timothy
- O'Sullivan-Beare, Daniel Robert
- Oulton, George Nugent
- Ovenden, Charles Thomas
- Ovens, Brig.-Gen. Robert Montgomery
- Owen-Lewis, Lieut.-Col. Arthur Francis
- Oxmantown, Lord