O
- O Antiphons
- O Deus Ego Amo Te
- O Filii Et Filiae
- O Salutaris Hostia
- Oates's Plot
- Oaths
- Oaxaca
- Obba
- Obedience
- Obedientiaries
- Oblate Sisters of Providence
- Oblates of Mary Immaculate
- Oblates of Saint Francis de Sales
- Oblati
- Obligation
- Obregonians
- Obreption
- Occasionalism
- Occasions of Sin
- Occult Art, Occultism
- Occult Compensation
- Occurence
- Octavarium Romanum
- Octave
- Odo of Canterbury
- Odo of Cheriton
- Odo of Glanfeuil
- Odoric of Pordenone
- Odorico Raynaldi
- Oecolampadius
- Oecumenius, Bishop of Trikka
- Oettingen
- Offa, King of Mercia
- Offerings
- Offertory
- Office of the Dead
- Ogdensburg
- Ogliastra
- Ohio
- Oil of Saints
- Ointment in Scripture
- Oklahoma
- Olaus Magnus
- Olba
- Old Catholics
- Old Hall (St. Edmund's College)
- Old Testament
- Oldenburg
- Olenus
- Olinda
- Oliva
- Oliver Almond
- Oliver Legipont
- Oliver Maillard
- Olivetans
- Olivier de La Marche
- Olmutz
- Olympe-Philippe Gerbet
- Olympus
- Omaha
- Ombus
- Omission
- Omnipotence
- Onias
- Onno Klopp
- Onofrio Panvinio
- Ontario
- Ontologism
- Ontology
- Ophir
- Oporto
- Oppido Mamertina
- Optimism
- Oracle
- Oran
- Orange Free State
- Orange River, South Africa
- Orans
- Orate Fratres
- Oratorio
- Oratory
- Orcagna
- Orcistus
- Ordeals
- Order and Abbey of Fontevrault
- Order of Aviz
- Order of Friars Minor
- Order of Friars Minor (Bibliography)
- Order of Friars Minor Conventuals
- Order of Gilbertines
- Order of Preachers
- Order of Saint James of Compostela
- Order of Saint Sylvester
- Order of Servites
- Order of St. Lazarus of Jerusalem
- Order of the Incarnate Word and Blessed Sacrament
- Order of the Knights of Christ
- Order of the Presentation
- Order of the Swan
- Order of Trinitarians
- Ordericus Vitalis
- Orders of Saint Anthony
- Orders of St. George
- Orders of the Holy Ghost
- Ordinariate
- Ordinary
- Ordines Romani
- Oregon
- Oregon City
- Oremus
- Orense
- Orestes Augustus Brownson
- Organ
- Oria
- Oriental Study and Research
- Orientation of Churches
- Orientius
- Oriflamme
- Origen and Origenism
- Original Sin
- Orihuela
- Oristano
- Orkneys
- Orlando de Lassus
- Orleans
- Oropus
- Orphans and Orphanages
- Orsini
- Orsisius
- Orthodox Church
- Orthodoxy
- Orthosias
- Ortolano Ferrarese
- Ortwin Gratius
- Orval
- Orvieto
- Osaka
- Osbald
- Osbern
- Osbern Bokenham
- Oscott (St. Mary's College)
- Osee
- Osimo
- Osma
- Osnabrueck
- Osorio Francisco Meneses
- Ossory
- Ostensorium
- Ostia and Velletri
- Ostiensis
- Ostracine
- Ostrogoths
- Otfried of Weissenburg
- Othlo
- Otranto
- Ottawa
- Ottmar Luscinius
- Otto I (The Great)
- Otto II
- Otto III
- Otto IV
- Otto of Blasien
- Otto of Freising
- Otto of Passau
- Otto Truchsess von Waldburg
- Ottobeuren
- Our Lady of Ambronay
- Our Lady of Arcachon
- Our Lady of Charity of the Good Shepherd
- Our Lady of Perpetual Succour (Our Lady of Perpetual Help)
- Our Lady of the Snow
- Overpopulation Theories
- Oviedo
- Owen Roe O'Neill
- Oxford
- Oxyrynchus
- Ozias
- St. Odilia
- St. Odilo
- St. Odo
- St. Olaf Haraldson
- St. Oliver Plunket
- St. Olympias
- St. Omer
- St. Optatus
- St. Osmund
- St. Oswald
- St. Oswald
- St. Oswin
- St. Othmar
- St. Ouen
- The Old Chapter
- The Oratory of Saint Philip Neri
- The Orders of the Annunciation
- The Organic Articles
- The Oxford Movement (1833-1845)