- Odon or Eudes of Cluny, Benedictine, abbot of Cluny
- Olympia of Constantinople, Widow and Deaconess
- Omer, Bishop of Therouanne
- Onesimus, Bishop of Ephesus and Martyr
- Oswald, Archbishop of York
- Our Lady, Help of Christians
- Our Lady of Fatima
- Our Lady of Good Counsel
- Our Lady of Guadalupe, Patroness of Latin America
- Our Lady of La Salette
- Our Lady of Lourdes, (1858)
- Our Lady of Mount Carmel
- Our Lady of Perpetual Help, (1863)
- Our Lady of Ransom or of Mercy
- Our Lady of the Rosary