Oliver Plunket, Saint - Biographical article on the professor of theology, Archbishop of Armagh and Primate of All Ireland, martyred in 1681. - http://www.newadvent.org/cathen/12169b.htm
Ozanam, Jacques - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11378b.htm
Ouen, Saint - Seventh-century Archbishop of Rouen, founder of several monasteries, devoted to theological studies, friend of St. Eloi. - http://www.newadvent.org/cathen/11360b.htm
Otto of Passau - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11359a.htm
Otto of Freising - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11358b.htm
Oswald, Saint - Archbishop of York, d. 992 while washing the feet of the poor, as was his daily custom during Lent. - http://www.newadvent.org/cathen/11348b.htm
Ostia and Velletri - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11346a.htm
Osmund, Saint - A Norman by birth, Osmund became bishop of Salisbury, and died in 1099. - http://www.newadvent.org/cathen/11340a.htm
Osma, Diocese of - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11339a.htm
Osee - The prophet and his book. - http://www.newadvent.org/cathen/11337a.htm
Origen and Origenism - Provides a biography and review of his works, as well as commentary on posthumous influences and the Origenistic Crises. - http://www.newadvent.org/cathen/11306b.htm
Orange Free State - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11267a.htm
Orange, Councils of - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11266b.htm
Optatus, Saint - Fairly long essay on the life and writings of this bishop, who argued against the Donatists. He died in about 387. - http://www.newadvent.org/cathen/11262b.htm
Oppido Mamertina - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11262a.htm
Omer, Saint - Swiss-born bishop of Terouenne in Belgic Gaul, founder of a monastery. He died in about 670. - http://www.newadvent.org/cathen/11251a.htm
Olympias, Saint - Widow, deaconess, renowned for her almsgiving, and a staunch supporter of St. John Chrysostom. - http://www.newadvent.org/cathen/11248b.htm
Olah, Nicolaus - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11234b.htm
Option, Right of - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11264a.htm
Oxford Movement, The (1833-1845) - 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. - http://www.newadvent.org/cathen/11370a.htm
Ojeda, Alonso de - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11230a.htm
Occasions of Sin - (Catholic Encyclopedia) - http://www.newadvent.org/cathen/11196a.htm