Oakeley, Frederick 
O Antiphons
Oates's Plot
Oaths, English Post-Reformation
Obazine, Monastery
Obba
Obedience
Oblate Sisters of Providence
Oblati, Oblatæ, Oblates
Obligation
Obregonians 
Obreption
O'Brien, Terence Albert
O'Carolan, Torlogh
Occasions of Sin
Occult Art, Occultism
O'Conor, Charles
Octavarium Romanum
O'Cullenan, Gelasius
Odense, Ancient See of Odense
O Deus Ego Amo Te
Odilia, Saint
Odo, Saint
Odo of Cambrai, Blessed
Œcumenius
Oettingen
Offa
Office, Divine
Office of the Dead
O'Fihely, Maurice
O Filii et Filiæ
O'Hely, Patrick
O'Herlahy, Thomas
O'Higgins, Ambrose and Bernard
Ohio
O'Hurley, Dermond
O'Hussey, Maelbrighte
Oil of Saints
Ointment in Scripture
Ojeda, Alonso de
Okeghem, Jean d'
Old Catholics
Old Chapter, The
Oldcorne, Ven. Edward
Olier, Jean-Jacques
Olivetans
O'Loghlen, Michael
Olympias, Saint
Olympus
Omer, Saint
Omission
Omnipotence
O'Neill, Hugh
O'Neill, Owen Roe
Onias
Ontologism
Ontology
Oostacker, Shrine of
Ophir
Oporto
Optatus, Saint
Option, Right of
O'Queely, Malachias
Oracle
Oran
Orate Fratres
Oratory, French Congregation
Orbellis, Nicolas d'
Ordeals
Orders, Holy
Oregon
Oregon City
O'Reilly, Bernard
Oresme, Nicole
Oria
Oriani, Barnaba
Oriental Study and Research
Orientation of Churches
Oriflamme
Origen and Origenism
Ory, Matthieu
Osbaldeston, Edward, Ven.
Osbern
Oscott 
Oslo, Ancient See
Osmund, Saint 
Ossat, Arnaud d'
Ossory, Diocese
Ostensorium
Ostracine
Ostraka, Christian
O'Sullivan Beare, Philip
Oswald, Saint -
Oswin, Saint
Othmar, Saint
Ottobeuren
Otto of St. Blasien
Ouen, Saint 
Our Lady of Good Counsel, Feast
Our Lady of the Fields, Brothers
Owen, Saint Nicholas
Oxford Movement, The
Ozanam, Antoine-Frédéric
Ozias