- O Antiphons
- O Blest Creator of the Light!
- O Bona patria, lumina sob ria te speculantur
- O Christ, the glory of the angel choirs
- O Christ, the World's Creator Bright
- O Christ, Thy Servants Spare!
- O Christ, Whose Glory Fills The Heaven
- O Day-Spring Bright
- O Deus, ego amo Te
- O Filil et Filiae
- O Gente felix hospita
- O Gloriosa virgin urn
- O Glorious King of Martyr Hosts
- O Glorious Lady! Throned on High
- O Glory of the Polish Race
- O God of Truth, O Lord of Might
- O God, come to my assistance
- O God, Of Those That Fought Thy Fight
- O God, Whose Hand Hath Spread The Sky
- O Great Creator of the Sky
- O Highest Hope of Mortals
- O House of Nazareth the Blest
- O Jesu, King Most Wonderful
- O Jesu, Thou The Beauty Art
- O Jesus, Jesus! dearest Lord
- O Kind Creator, Bow Thine Ear
- O Light of Light
- O Lovely Light of Fatherland!
- O Lux beata caelitum
- O Lux beata Trinitas
- O More Than Blessed, Merit High Attained
- O Nimis felix, meritique celsi
- O Quot undis lacrimarum
- O Rosy Dawn! That Dost Proclaim
- O Sacred Heart!
- O Salutaris Hostia
- O Saving Victim, Opening Wide
- O Sol salutis, intimis
- O Sola magnarum urbium
- O Sovereign Lord of Nature's Might
- O Splendor of God's Glory Bright
- O Strength, and Stay Upholding All Creation
- O Thou, The Heavens' Eternal King
- O Trinity of Blessed Light
- O Turn to Jesus, Mother! Turn
- O.A.R.
- O.C.
- O.C.C.
- O.C.D.
- O.C.R.
- O.C.S.O.
- O.Camald.
- O.Carm.
- O.Cart.
- O.Cist.
- O.de.M.
- O.F.M.
- O.F.M.Cap.
- O.F.M.Conv.
- O.F.M.I.
- O.F.M.Reg.
- O.H.
- O.L.M.
- O.L.M.E.
- O.L.V.M.
- O.M.
- O.M.C.
- O.M.Cap.
- O.M.I.
- O.M.M.
- O.M.V.
- O.Merced.
- O.P.
- O.Pr.
- O.Praem.
- O.R.C.
- O.R.S.A.
- O.S.
- O.S.A.
- O.S.B.
- O.S.B.M.
- O.S.C.
- O.S.Cam.
- O.S.F.
- O.S.F.N.
- O.S.F.S.
- O.S.H.
- O.S.J.
- O.S.M.
- O.S.P.P.E.
- O.Ss.R.
- O.Ss.S.
- O.Ss.T.
- O.S.T.R.
- O.S.U.
- O.T.
- O.Trinit.
- O'Brien, William
- O'Callaghan, Edmund Bailey
- O'Carolan, Turlogh
- O'Connell, Daniel
- O'Connell, John
- O'Conor, Charles 12th century
- O'Conor, Charles 19th century
- O'Curry, Eugene
- O'Donovan, John
- O'Dwyer, Joseph
- O'Growney, Eugene
- O'Hagan, John
- O'Hagan, Thomas
- O'Hanlon, John
- O'Higgins, Ambrose Bernard
- O'Higgins, Bernardo
- O'Loghlen, Michael, Sir
- O'Meara, Kathleen
- O'Reilly, Edmund
- O'Reilly, John Boyle
- O'Reilly, Myles William Patrick
- O'Toole, Lawrence, Saint
- Oak, Abraham's
- Oakland, California, diocese of
- Oakley, Frederick
- Oates, Titus
- oath
- Oaths, English Post Reformation
- Obadiah
- obedience
- obedience, religious
- obedientiaries
- Oberammergau, Passion Play of
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- obituaries
- oblate
- Oblate Sisters of Providence
- Oblates of Mary Immaculate
- Oblates of Saint Ambrose
- Oblates of Saint Charles
- Oblates of Saint Francis de Sales of Troyes
- oblates, orders of
- oblation
- obligation
- obreption
- Observants
- Observatory, Manila
- obstinacy
- occasion of sin
- Occille of Barcelona, Saint
- occult art
- occultism
- Oceania
- Ockenheim, Jean d'
- oct.
- octaves
- Octavius, Pope
- Octavius - anti-pope
- October
- Odollam
- Odd Fellows, Order of
- Oddone Colonna
- Odilo, Saint
- Odington, Walter
- odium theologicum
- Odoric of Pordenone, Blessed
- OEcolampadius, Johann
- oecumenical councils
- Oersted, Hans Christian
- Oetinga
- Oettingen
- Offense, Mount of
- offertory
- Office of the Dead
- office, abuse of
- Office, Divine
- offices, Passion
- Og
- Ogdensburg, New York, diocese of
- Ogilvie, John, Saint
- Oh come and mourn with me a while
- Ohio
- Ohm, Georg Simon
- oil of catechumens
- oil of saints
- oil of the sick
- oil stock
- oil, olive
- Oils, Holy
- ointment
- Okeghem, Jean d'
- Oklahoma
- Oklahoma City, Oklahoma, archdiocese of
- Olacie of Barcelona, Saint
- Olaf Haraldson, Saint
- Olaille of Barcelona, Saint
- Olaire of Barcelona, Saint
- Olaus Magnus
- Old Catholic Church in America
- Old Catholics
- Old Latin Version
- Old Law
- Old Order German Baptist Brethren
- Old Sarum
- Old School Baptists
- Old Testament, sacrifices in the
- Olier, Jean Jacques
- Olivaint, Pierre
- olive branch
- olive oil
- Olive Sunday
- Oliver Cromwell
- Oliver Plunket
- Olivet, Mount
- Omaha, Nebraska, archdiocese of
- omega, alpha and
- Omer, Saint
- omission
- omnipotence
- Omnis expertem macule Mariam
- omniscience
- omophorion
- onanism
- Once in Royal David's City
- Onesimus
- Onias
- Onitsha, Nigeria, archdiocese of
- Only Begotten Son
- Onolatry
- ontologism
- Open Brethren
- Opes decusque regium reliquetas
- opinion
- Optatus, Saint
- optimism
- opus operatum
- oracle - garment
- oracle - communication
- Orange, California, diocese of
- Orange, Council of
- Orangemen
- Orans
- Orante
- orate fratres
- Orate Fratres
- Oratorians
- oratorio
- oratory
- Oratory of Saint Philip Neri
- Ord.Fratr.Praed.
- Ord.Praem.
- ordeals
- order
- Order de Santiago de la Espada
- Order of Aviz
- Order of Calced Carmelites
- Order of Canons Regular of Prémontré
- Order of Citeaux
- Order of Clerks Regular
- Order of Discalced Carmelites
- Order of Fontevrault
- Order of Friars Minor
- Order of Friars Minor Capuchins
- Order of Friars Minor Conventuals
- Order of Friars Preachers
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- Order of Knights of Saint Gregory
- Order of Minims
- Order of Our Lady of Bethlehem
- Order of Our Lady of Mercy for the Ransom of Captives
- Order of Our Lady of Mount Carmel
- Order of Pius IX
- Order of Preachers
- Order of Reformed Cistercians of Our Lady of La Trappe
- Order of Saint Benedict
- Order of Saint Dominic
- Order of Saint James of Compostela
- Order of Saint Lazarus of Jerusalem
- Order of Saint Saviour
- Order of Saint Sylvester
- Order of Servants of Mary
- Order of the Annunziata
- Order of the Golden Militia
- Order of the Golden Spur
- Order of the Holy Ghost
- Order of the Knights of Christ
- Order of the Mass
- Order of the Most Holy Trinity
- Order of the Presentation
- Order of the Servants of the Sick
- Order of the Visitation
- Order, Camaldolese
- Order, Carmelite
- Order, Carthusian
- Order, Crosier
- order, Ionic
- Order, Seraphic
- orders of knighthood
- orders of oblates
- Orders, Anglican
- orders, hierarchy of
- Orders, Holy
- Orders, impediment of
- Orders, Minor
- ordinary
- Ordinary of the Mass
- ordination
- Ordines Romani
- ordo
- Oregon
- oremus
- Oresme, Nicole
- Orestes Augustus Brownson
- organ
- organic articles
- Oriani, Barnaba
- Oriental Church, Congregation for the
- orientation
- oriflamme
- Origen
- Origenism
- original justice
- original sin
- Orkney, Scotland, diocese of
- Orlando Bandinelli
- Orlando, Florida, diocese of
- Orlandus de Lassus
- Ormuzd
- orphanages
- orphans
- Orsini
- Ortelius, Abraham
- Orthodox Church
- Orthodox Society of Friends
- Orthodoxy
- Osbern Bokenham
- Oscott
- osculatorium
- Osee, Book of
- Osmund, Saint
- Ossory, Ireland, diocese of
- ostiarius
- ostensorium
- ostraka
- Oswald, Saint
- Oswald of Northumbria, Saint
- Oswald Beer
- Oswin, Saint
- Othmar, Saint
- Ottawa, Ontario, archdiocese of
- Ottawa, University of
- Otto, Saint
- Otto I, Emperor
- Otto II, Emperor
- Otto III, Emperor
- Otto IV, Emperor
- Otto Eduard Leopold von Bismarck
- Otto the Great
- Ottobuono Fieschi
- Ouen, Saint
- Our Father
- Our Lady of Good Counsel
- Our Lady of Guadalupe, Feast of
- Our Lady of Loreto
- Our Lady of Lourdes
- Our Lady of Mercy, Feast of
- Our Lady of Mount Carmel, Feast of
- Our Lady of Perpetual Help
- Our Lady of Ransom, feast of
- Our Lady of Ransom, scapular of
- Our Lady of the Elms, College of
- Our Lady of the Holy Rosary, Seminary of
- Our Lady of the Lake College
- Our Lady of the Lake, Seminary of
- Our Lady of the Pillar
- Our Lady of the Snow, Feast of
- Our Lady of Victory, Feast of
- Our Lady, altar of
- Our Lady's Dowry
- Our Limbs Refreshed With Slumber Now
- Our Lord, names of
- Our Missions
- Our Savior, Apparition of
- Our Souls' Great Teacher Paul, Our Guide in Wisdom's Ways
- Our Sunday Visitor
- Out of the depths
- Overbeck, Johann Friedrich
- Owen, Saint
- Owensboro, Kentucky, diocese of
- ox
- Oxford Catholic Worker's College
- Oxford Movement
- Oxford, England
- Oxford, University of
- Ozanam, Antoine
- Ozanam, Jacques
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