| Saint | Background | Date | Feast Day |
| Obdulia | of Toledo | 05/09 | |
| Obertus | 15/12 | ||
| Octavianus | Martyr at Carthage | C5th | |
| Oda | 23/10 | ||
| Oda | 27/11 | ||
| Oda | Virgin | 21/11 | |
| Odarnat/Orna | 13/11 | ||
| Odger | 08/05 | ||
| Odhran/Odran | of Ireland (Pádraig's Charioteer) | C5th | 19/02 |
| Odhran | 06/03 | ||
| Odhran | 18/08 | ||
| Odhran | 10/09 | ||
| Odhran | 02/10 | ||
| Odhran/Oran/Odharan | of Meath | d563 | 22/10 |
| Odhran | 26/10 | ||
| Odhran/Odran/Odrian/Otteran | Abbot/Bishop of Waterford | d563 | 27/10 08/05 |
| Odilia/Odilda/Odile/Odille/Ottilie/Odette | Abbess, (Alsace) Niedermunster | d c720 | 13/12 15/12 |
| Odilo | of Chartres | 12/04 | |
| Odilo | of Stavelot | 15/10 | |
| Odilo | Abbot of Cluny | C10th | 19/11 18/11) |
| Odo | of Novaria | 14/01 | |
| Odo/Oda the Good | Archbishop of Canterbury | 02/06 04/07 | |
| Odo | 2nd Abbot of Cluny | C10-11th | 01/01 (02/01) |
| Odoric | of Pordenone | 28/01 | |
| Odorius | Martyr | 15/05 | |
| Odulph | 18/07 | ||
| Odulph/Odulf | 12/06 | ||
| Odulvald | Abbot of Maelros/Melrose | 26/05 | |
| Offa | of Essex | 15/12 | |
| Offren/Vulfran | 18/03 | ||
| Ogerius | of Locerio | 10/09 | |
| Oisin | 01/01 | ||
| Oisin | 01/05 | ||
| Oisin | 19/07 | ||
| Oisin | 22/07 | ||
| Olaf | King of Norway | d1030 | 29/07 (03/08) |
| Olaf | King of Sweden | d c950 | 29/07 |
| Olga | wife of Igor of Kiev | 879-969 | 11/07 |
| Olimpiadis | 15/04 | ||
| Olive | 03/06 | ||
| Oliver Plunkett | Archbishop of Armagh, executed at Tyburn | 1629-81 | 11/07 (01/07) |
| Olivia | 10/07 | ||
| Olivia/Olive, Blessed | of Palermo, martyred in Tunis | C9th | 10/06 |
| Olivia/Olive | virgin martyr | C1st/2nd | 05/03 |
| Olympias | Nun, Constantinople | C4th | 17/12 |
| Omer | 17/09 | ||
| Omer | of Therouanne | 09/09 | |
| Onesimos/Onesimus | 15/02 | ||
| Onesimus | Bishop of Soissons | 12/05 | |
| Onesiphorus | 06/09 | ||
| Onouphrios | of Gabrovo | d1818 | (Orth) 04/01 |
| Onuphrius/Onouphrius | monk/hermit near Thebes | C4th/C5th | 11/06 (10/06 12/06 13/06) |
| Opportuna | 22/04 | ||
| Optatus | 04/06 | ||
| Optatus | 27/11 | ||
| Optatus | Bishop of Auxerre | 31/08 | |
| Orientius | Bishop of Auch | 01/05 | |
| Oringa | 04/01 | ||
| Orora/Orara | Isle of Man | 20/10 | |
| Osan | 15/10 | ||
| Osana | of Mantua | b1449 | 20/06 |
| Osburga | Abbess of Coventry | 30/03 | |
| Oscar | 03/02 | ||
| Osith(a)/Osyth | of Essex | c700 | 07/10 |
| Osmanna | of Ireland/Brieuc | 09/09 | |
| Osmund/Osmond | Bishop of Salisbury | d1099 | 04/12 16/07 |
| Osonans | 08/08 | ||
| Oswald | Bishop of Worcester & Archbishop of York | d992 | 28/02 (23/02 Trans 15/10) |
| Oswald | King of Northumbria | d641 | 05/08 04/08 (08/08)Trans 20/06 |
| Oswin | King of Deira | C7th | 20/08 Trans 11/03 |
| Otger/Othger | of Northumbria | C8th | 10/09 |
| Othelric | 04/07 | ||
| Othmar/Otmar | Abbot of St Gall | d759 | 17/11 (16/11) |
| Otto | Bishop of Bamberg | 1062-1139 | 02/07 01/10 Trans 30/09 |
| Oubertus | 12/12 | ||
| Oudoc(eus) | Bishop of Landaff | C7th | 02/07 |
| Owen | hermit of Lastingham & Lichfield | C7th | 04/03 03/03 |
| Owen | Translation 05/05 | ||
| Owen/Ouen/Oeun/Audoenus/Dado | Archbishop of Rouen | c600-84 | 24/08 |
This collection of names was compiled by Kate Monk and is ©1997, Kate Monk.
Copies may be made for personal use only.