O’Carroll, O’Carrolls, [309], [323], [352]
O’Colan, Dominick, [423], [424], [462]
O’Connor, Brian MacGilpatrick, Teig MacGilpatrick, Connor MacCormac, Morrogh ne Cogge, all of Offaly, [121], [122]
— Roe, in Roscommon, [363], [365]
— Sligo, Sir Donnell, [43], [60], [147], [208]
— Cahil Oge, brother of the foregoing, [208]
— Sligo, Donough, son of Cahil, [208], [209], [279], [284], [336], [338], [365], [384], [427], [429], [432]
O’Connor, Kerry, [378], [406], [431], [443]
— Eugene, Bishop of Killala (not Killaloe), [459]
— Dermot, leader of free companions, [360], [364], [366], [383], [384]