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This list represents only a tiny fraction of articles available on the New Advent website. For a more complete list, please see the full index for O or use the search box at the top of this page.

O Antiphons - Seven antiphons to the Magnificat, used in the Divine Office in the days preceding Christmas
Oates's Plot - A 'Popish Plot' which, during the reign of Charles II of England, Titus Oates pretended to have discovered.
Oaths - An invocation to God to witness the truth of a statement
Oaths, English Post-Reformation - The English Reformation having been imposed by the Crown, it was natural that submission to the essential points of its formularies should have been exacted with some solemnity, by oath, test, or formal declaration . . .
Obedience - The complying with a command or precept.
Obedience, Religious - The general submission which religious vow to God, and voluntarily promise to their superiors, in order to be directed by them in the ways of perfection according to the purpose and constitutions of their order.
Obedientiaries - The lesser officials of a monastery who were appointed by will of the superior.
Oblates of Mary Immaculate - Religious society founded in 1816
Obligation - Defined in the 'Institutes' of Justinian as a 'legal bond which by a legal necessity binds us to do something according to the laws of our State'
Obreption - A type of fraud by which an ecclesiastical rescript is obtained
Observatory, Vatican - Gregory XIII ordered a tower to be erected in a convenient part of the Vatican buildings, and to be fitted out with the greatest and best instruments of the time
Occam, William of - Biographical article on the fourteenth-century Franciscan philosopher
Occasionalism - The metaphysical theory which maintains that finite things have no efficient causality of their own, but that whatever happens in the world is caused by God, creatures being merely the 'occasions' of the Divine activity.
Occasions of Sin - External circumstances — things or persons — which incite or entice one to sin.
Occult Art, Occultism - An attempt to work apparent miracles not by the power of God, but by the use of hidden forces beyond man's control
Ockham, William of - Biographical article on the fourteenth-century Franciscan philosopher
O'Connell, Daniel - Irish patriot (1775-1847)
Octave - A period of eight days, often observed liturgically
Odilia, Saint - Patroness of Alsace, d. 720, founded the convent of Hohenburg
Œcolampadius, John - Protestant theologian and organizer of Protestantism at Basle (1482-1531)
Offerings - Covers the Jewish and Christian practices of offerings and tithes
Offertory - The rite by which the bread and wine are presented to God before they are consecrated
Office, Divine - Brief essay on the historical development of the Liturgy of the Hours
Office of the Dead - Historical article on a special set of prayers for the deceased
O'Higgins, Ambrose and Bernard - South American patriots of Irish background
Ohio - The seventeenth state of the American Union
Oil of Saints - An oily substance which flows from the relics or burial places of certain saints
Oils, Holy - Oil is a product of great utility the symbolic signification of which harmonizes with its natural uses. It serves to sweeten, to strengthen, to render supple; and the Church employs it for these purposes in its rites
Ointment in Scripture - That the use of oily, fragrant materials to anoint the body is a custom going back to remote antiquity is evidenced by the Old Testament as well as other early literatures. . .
Olaf Haraldson, Saint - Martyr and King of Norway, d. 1030
Old Catholics - The sect organised in German-speaking countries to combat the dogma of Papal Infallibility.
Old Testament - The Apostle St. Paul declares himself (II Cor., iii, 6) a minister 'of the new testament', and calls (iii, 14) the covenant entered into on Mount Sinai 'the old testament'
Old Testament, Canon of the - Signifies the authoritative list or closed number of the writings composed under Divine inspiration, and destined for the well-being of the Church
Olier, Jean-Jacques - Founder of the seminary and Society of St-Sulpice (1608-1657)
Olivet, Mount - A hill immediately east of Jerusalem
Olivetans - A branch of the white monks of the Benedictine Order, founded in 1319.
Omission - The failure to do something one can and ought to do
Omnipotence - The power of God to effect whatever is not intrinsically impossible
Ontologism - An ideology which maintains that God and divine ideas are the first object of our intelligence and the intuition of God the first act of our individual knowledge
Ontology - An article on 'the science of being'
Opening Prayer (in the Mass) - The name now used only for short prayers before the Epistle in the Mass, which occur again at Lauds, Terce, Sext, None, and Vespers
Ophir - A people and a country mentioned in the Bible
Optimism - A metaphysical theory or an emotional disposition.
Oracle - A Divine communication given at a special place through specially appointed persons; also the place itself.
Orange, Councils of - Two councils that were held in southern France
Orans - An iconographic type, commonly found in the Roman catacombs, of a female figure praying with extended arms
Orate Fratres - The exhortation addressed by the celebrant to the people before the Secrets in the Roman Mass
Oratorio - A musical composition for solo voices, chorus, orchestra, and organ, to a religious text generally taken from Holy Scripture.
Oratory - A structure other than a parish church, set aside by Church authority for prayer and the celebration of Mass
Oratory of Saint Philip Neri, The - Italian, Spanish, English, and other communities, which follow the rule of St. Philip Neri.
Ordeals - A means of obtaining evidence by trials, through which the guilt or innocence of an accused person was supposedly established
Orders, Holy - The sacrament by which grace and spiritual power for the discharge of ecclesiastical offices are conferred.
Orders, The Military - A historical review of dozens of military orders
Ordinariate - This term is used in speaking collectively of all the various organs through which an ordinary, and especially a bishop, exercises the different forms of his authority.
Ordinary - Denotes any person possessing or exercising ordinary jurisdiction
Ordines Romani - The brief conspectus of the daily Office and Mass as adapted to the local calendar
Oremus - Invitation to pray, said before collects and other short prayers and occurring continually in the Roman Rite.
Oresme, Nicole - Article by Pierre Duhem on this medieval scientific thinker
Organ - A musical instrument which consists of one or several sets of pipes, each pipe giving only one tone, and which is blown and played by mechanical means.
Organic Articles, The - A name given to a law regulating public worship, comprising 77 articles relative to Catholicism, and 44 relative to Protestantism, presented by order of Napoleon to the Tribunate and the legislative body at the same time that he made these two bodies vote on the Concordat itself
Orientation of Churches - According to Tertullian the Christians of his time were, by some who concerned themselves with their form of worship, believed to votaries of the sun. . .
Oriflamme - Legendary banner
Origen and Origenism - Provides a biography and review of his works, as well as commentary on posthumous influences and the Origenistic Crises
Original Sin - Original sin is (1) the sin that Adam committed; (2) a consequence of this first sin, the hereditary stain with which we are born on account of our origin or descent from Adam.
Orléans, Councils of - Six national councils held in the Merovingian period.
Orphans and Orphanages - The death of one or both parents makes the child of the very poor a ward of the community. . .
Orsini - One of the most ancient and distinguished families of the Roman nobility, whose members often played an important role in the history of Italy
Orthodox Church - The body of Christians in union with the Patriarch of Constantinople but in schism with the Pope of Rome.
Orthodoxy - Right belief or purity of faith.
Orthodoxy, Feast of - Eastern feast that commemorates the restoration of the holy icons to the churches in 842
Orvieto - Diocese in central Italy
O Salutaris Hostia - Hymn used for the Benediction of the Most Blessed Sacrament
Osee - The prophet and his book
Ostensorium - A vessel designed for the exhibition of some object of piety
Ostrogoths - One of the two chief tribes of the Goths
Oswald, Saint - King of Northumbria and martyr, d. 642
O'Toole, Saint Lawrence - Confessor, abbot, and the first Irish-born bishop of Dublin, d. 1180
Otto I, the Great - Roman emperor and German king (912-973)
Our Father, The - Although the Latin term oratio dominica is of early date, the phrase 'Lord's Prayer' does not seem to have been generally familiar in England before the Reformation. During the Middle Ages the 'Our Father' was always said in Latin, even by the uneducated. Hence it was then most commonly known as the Pater noster
Our Lady, Help of Christians, Feast of - The invocation 'Help of Christians' originated in the sixteenth century.
Our Lady of Charity of the Good Shepherd - The aim of this institute is to provide a shelter for girls and women of dissolute habits, who wish to do penance for their iniquities and to lead a truly christian life
Our Lady of Good Counsel, Feast of - Feast honoring a popular Marian devotion
Our Lady of the Snow - Feast commemorating the dedication of the church of Santa Maria Maggiore in Rome
Our Lady of Perpetual Help - A famous Byzantine-style picture painted on wood, with a background of gold
Our Lady of Perpetual Succour - A famous Byzantine-style picture painted on wood, with a background of gold
Overpopulation, Theories of - Down to the end of the eighteenth century, very little attention was given to the relation between increase of population and increase of subsistence. . .
Owen, Saint Nicholas - A Jesuit lay brother, skilled in building hiding places for priests. He died under torture in 1606
Oxford, University of - Famous institution in England
Oxford Movement, The - Lengthy historical article on the Tractarian Movement includes information on John Henry Newman, as well as on other leading lights of this nineteenth-century Anglo-Catholic campaign, such as John Keble and Hurrell Froude
Ozanam, Antoine-Frédéric - Great grand-nephew of Jacques Ozanam (1813-1853)
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