Octavian, Archdeacon of Carthage, Martyr
Octavius, Martyr, Patron of Turin
Oda, Widow, Foundress of St. George Church and hospice at Hamay-les-Marchiennes
Odilia, (Ottilia), abbess of Odielienburg, Patroness of Alsace
Odilo, Abbot of Cluny
Odilo, Abbot of Stavelot and Malmedy
Odo (Oda), Archbishop of Canterbuty
Odo, Bishop of Urgel
Odrada, Virgin, of Haelen (Hertogenbosch), Netherlands
Odulph, Canon of Utrecht
Oduvald, Abbot of Melrose
Olaf, King of Norway
Olegarius, Bishop of Barcelona
Olga, Princess of Kiev
Oliver Plunkett, Archbishop of Armagh and Martyr
Olivia, Virgin, Martyred at Palermo
Olympias of Constantinople, Widow and Deaconess
Olympias, Consular Prefect of Ameria, Martyr
Olympius, Tribune and Martyr
Onesimus, Philemon's escaped Slave, second Bishop of Ephesus, martyr
Onuphrius (Humphrey) “the Great”, Hermit
Opilius of Piacenza, Martyr
Opportuna, Abbess of Montreuil (Benedictine), sister of Chrodegand of Seez
Orencius, Father of St. Lawrence of Rome
Orestes of Tyana, Cappadocia, Physician and Martyr
Ortarius, Abbot of Landelles
Osanna Andreassi of Mantua, Dominican Nun
Osburga, Abbess of Coventry
Osmund, Bishop of Old Sarum (Salisbury)
Oswald, Bishop of Worcester, Archbishop of York
Oswald, King of Northumbria, Martyr
Oswin, King of Northumbria and Martyr
Otto, Bishop of Bamburg
Ouen (Owen, Audoenus), Archbishop of Rouen