“ Oldisworth, Giles Oldisworth, Michael Oldisworth, William Oldmixon, John Oldys, Valentine Oldys, William O'Leary, Arthur O'Leary, Ellen O'Leary, Joseph Oley, Barnabas Olifard, William Oliphant, Carolina Oliphant, Francis Wilson Oliphant, James Oliphant, Laurence (d.1500?) ”

“ Ormerod, George Wareing Ormerod, Oliver Ormerod, William Piers Ormesby, William de Ormonde, James Ornsby, George Ornsby, Robert O'Rourke, Brian-na-Murtha O'Rourke, Tiernan Orr, Hugh Orr, James Orr, John Orr, William Orridge, Benjamin Brogden Ortelius, Abraham Orton, Job Orton, Reginald Orum, John Osbald Osbaldeston, George Osbaldeston, Lambert Osbaldeston, Richard Osbaldeston, William Osberht Osbern (fl.1090) Osbern (d.1103) Osbern, Claudianus Osborn Wyddel Osborn, Elias Osborn, George Osborn, John Osborn, Robert Durie Osborn, Sherard Osborne, Dorothy. ”

“ Olivers, Thomas Ollier, Charles Ollier, Edmund Olliffe, Joseph Francis Ollivant, Alfred Ollyffe, John O'Lochlainn, Domhnall O'Lochlainn, Muircheartach O'Loghlen, Colman Michael O'Loghlen, Michael O'Lothchain, Cuan O'Maelchonaire, Fearfeasa O'Mahony, Connor (fl. 1650) . ”