| Celebration | Name | Saint | Birth | Death |
|---|---|---|---|---|
| 1 of January | Odilón | Saint Odilón of Cluny | 1049 France | |
| 14 of January | Odo | Blessed Odo of Novara | 1200 Italy | |
| 14 of January | Odoric | Blessed Odoric of Pordenone Mattiuzzi | 1331 Italy | |
| 16 of January | Otón | Saint Otón | 1226 Morocco | |
| 28 of January | Olimpia | Blessed Olympia Bida | 1952 Russian federation | |
| 3 of February | Oscar | Saint Ansgar of Bremen | 865 Germany | |
| 3 of February | Olivia | Saint Olivia mártir | ||
| 15 of February | Onesimus | Blessed Onesimus | ||
| 28 of February | Osvaldo | Saint Osvaldo | 992 United kingdom | |
| 6 of March | Olegario | Saint Olegarius | 1137 Spain | |
| 11 of March | Oengo | Saint Oengo Cúldeo | 824 Ireland | |
| 23 of March | Otón | Saint Otón of Ariano | 1120 Italy | |
| 30 of March | Osburga | Saint Osburga | 1018 United kingdom | |
| 15 of April | Ortario | Saint Ortario of Landelles | France | |
| 16 of April | Optato | Saint Optato y compañeros mártires | Spain | |
| 22 of April | Oportuna | Saint Oportuna | 770 France | |
| 1 of May | Orencio | Saint Orencio | 440 France | |
| 30 of May | Otto | Blessed Otto Neururer | 1882 Austria | 1940 Germany |
| 3 of June | Oliva | Saint Oliva of Anagni | Italy | |
| 4 of June | Optato | Saint Optato of Milevi | Algeria | |
| 12 of June | Onofre | Saint Onophrius of Egypt | Egypt | Egypt |
| 12 of June | Odulfo | Saint Odulfo of Utrecht | Netherlands | 855 Netherlands |
| 12 of June | Olimpio | Saint Olimpio of Tracia | Turkey | |
| 30 of June | Ostiano | Saint Ostiano of Viviers | France | |
| 1 of July | Oliverio | Saint Oliverio Plunkett | 1681 United kingdom | |
| 2 of July | Otho | Saint Otho of Bamberg | 1060 Germany | 1139 Germany |
| 7 of July | Odón | Saint Odón of Urgel | 1122 Spain | |
| 7 of July | Oddino | Blessed Oddino Barotti | 1400 Italy | |
| 11 of July | Olga | Saint Olga of Kiev | 969 Ukraine | |
| 14 of July | Optaciano | Saint Optaciano of Brescia | Italy | |
| 25 of July | Olimpíada | Saint Olimpíada of Nicomedia | 408 Greece | |
| 29 of July | Olav | Saint Olav of Noruega | 1030 Norway | |
| 4 of August | Onofre | Saint Onofre eremita | 995 Italy | |
| 6 of August | Octavian | Blessed Octavian of Savona | 1132 Italy | |
| 9 of August | Osvaldo | Saint Oswald of Oswestry | 642 United kingdom | |
| 6 of September | Onesíforo | Saint Onesíforo of Éfeso | ||
| 10 of September | Ogier | Blessed Ogier of Locedio | 1214 Italy | |
| 18 of September | Océano | Saint Océano of Nicomedia | Turkey | |
| 22 of September | Otto | Blessed Otto of Morimond | 1158 France | |
| 3 of October | Otto | Blessed Otto of Metten | 802 Germany | |
| 12 of October | Opilio | Saint Opilio of Piacenza | Italy | |
| 17 of October | Oseas | Saint Oseas profeta | Israel | Israel |
| 27 of October | Oterano | Saint Oterano of Iona | United kingdom | |
| 3 of November | Odrada | Saint Odrada of Alem | Belgium | |
| 6 of November | Onofre | Blessed Onofre | Spain | Spain |
| 6 of November | Otilia | Blessed Otilia Alonso González | Spain | Spain |
| 6 of November | Ovidio | Blessed Ovidio Beltrán | Spain | Spain |
| 6 of November | Olegario | Blessed Olegario Ángel | Spain | Spain |
| 9 of November | Orestes | Saint Orestes of Capadocia | Turkey | Turkey |
| 10 of November | Orestes | Saint Orestes of Tiana | Turkey | |
| 16 of November | Otmaro | Saint Otmaro of Suiza | Switzerland | 759 Germany |
| 18 of November | Odón | Saint Odo of Cluny | France | 942 France |
| 20 of November | Octavio | Saint Octavio of Turín | Italy | 286 Italy |
| 20 of November | Octaviano | Saint Octaviano | Italy | Italy |
| 2 of December | Oderisio | Blessed Oderisio | ||
| 4 of December | Osmundo | Saint Osmundo | United kingdom | 1099 United kingdom |
| 6 of December | Obicio | Saint Obicio | Italy | 1204 Italy |
| 13 of December | Orestes | Saint Orestes | ||
| 13 of December | Otilia | Saint Otilia | France | France |