| Bishops |
 Bishop Matthais Ob
| Titular Bishop of Daulis (1566.06.10 – 1572) Auxiliary Bishop of Speyer (Germany) (1566.06.10 – 1572) |
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  Bishop Ernest Obodo (57)
Titular Bishop of Mediana
Auxiliary Bishop of Enugu (Nigeria)
| Born: | 1966.10.24 (Awha-Imezi, Nigeria) | | Ordained Priest: | 2000.07.22 | | Consecrated Bishop: | 2018.08.31 |
| Priest of Enugu (Nigeria) (2000.07.22 – 2018.05.25) Titular Bishop of Mediana (2018.05.25 – ...) Auxiliary Bishop of Enugu (Nigeria) (2018.05.25 – ...) |
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 Bishop Mikołaj Oborski
| Ordained Priest: | 1636 | | Consecrated Bishop: | 1658.08.04 | | Died: | 1689.04.16 |
| Titular Bishop of Laodicea (1658.03.18 – 1689.04.16) Auxiliary Bishop of Kraków (Poland) (1658.03.18 – 1689.04.16) |
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 Bishop Claude-Thierry Obré
| Born: | 1806.09.04 (France) | | Consecrated Bishop: | 1878.01.15 | | Died: | 1881.12.14 († 75) |
| Auxiliary Bishop of Beauvais (France) (1877.12.14 – 1878.02.28) Titular Bishop of Zoora (1877.12.14 – 1881.12.14) |
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 Bishop Sebastián Obregón, O.S.B.
| Born: | (Spain) | | Consecrated Bishop: | 1535.05.01 | | Died: | 1559.01.08 |
| Titular Bishop of Marocco (1534.12.02 – 1539?) Auxiliary Bishop of Sevilla (Spain) (1534.12.02 – 1559.01.08) |
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 Bishop James Joseph O’Brien
| Born: | 1930.08.05 (England) | | Ordained Priest: | 1954.06.12 | | Consecrated Bishop: | 1977.09.21 | | Died: | 2007.04.11 († 76) |
| Auxiliary Bishop of Westminster (England) (1977.06.28 – 2005.06.30) Titular Bishop of Manaccenser (1977.06.28 – 2007.04.11) |
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Bishop Thomas Kevin O’Brien | Born: | 1923.02.18 (Ireland) | | Ordained Priest: | 1948.06.20 | | Consecrated Bishop: | 1981.12.08 | | Died: | 2004.12.27 († 81) |
| Auxiliary Bishop of Middlesbrough (England) (1981.11.09 – 1998.06.25) Titular Bishop of Ardcarne (1981.11.09 – 2004.12.27) |
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 Archbishop William David O’Brien
| Born: | 1878.08.03 (Chicago, IL, USA) | | Ordained Priest: | 1903.07.11 | | Consecrated Bishop: | 1934.04.25 | | Died: | 1962.02.19 († 83) |
| Titular Bishop of Calynda (1934.02.10 – 1953.11.18) Auxiliary Bishop of Chicago (USA) (1934.02.10 – 1962.02.19) Titular Archbishop of Calynda (1953.11.18 – 1962.02.19) |
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Archbishop Henry O’Callaghan |
Bishop Francisco Ocampo, O.S. | Consecrated Bishop: | 1660.10.03 |
| Titular Bishop of Amyclae (1660.06.21 – ?) |
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 Archbishop William Dominic O’Carroll, O.P.
| Born: | 1826.03.26 (Ireland) | | Ordained Priest: | 1848.04.28 | | Consecrated Bishop: | 1874.05.24 | | Died: | 1880.10.13 († 54) |
| Titular Archbishop of Alabanda (1874.02.03 – 1880.10.13) Coadjutor Archbishop of Port of Spain (Trinidad and Tobago) (1874.02.03 – 1880.10.13) |
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 Bishop Fiachra Ó Ceallaigh, O.F.M.
Dia ár misneach | Born: | 1933.08.18 (Ireland) | | Ordained Priest: | 1961.07.02 | | Consecrated Bishop: | 1994.09.17 | | Died: | 2018.07.29 († 84) |
| Auxiliary Bishop of Dublin (Ireland) (1994.06.07 – 2009.09.17) Titular Bishop of Tre Taverne (1994.06.07 – 2018.07.29) |
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Bishop David Gerard O’Connell Jesus I trust in you | Born: | 1953.08.16 (Cork, Ireland) | | Ordained Priest: | 1979.06.10 | | Consecrated Bishop: | 2015.09.08 | | Died: | 2023.02.18 (Hacienda Heights, CA, USA † 69) |
| Priest of Los Angeles (USA) (1979.06.10 – 2015.07.21) Prelate of Honour of His Holiness (1999 – 2015.07.21) Titular Bishop of Cell Ausaille (2015.07.21 – 2023.02.18) Auxiliary Bishop of Los Angeles (USA) (2015.07.21 – 2023.02.18) Murdered by gunshot. |
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Archbishop Denis Joseph O’Connell Et juste et vray |
 Bishop Joseph Peter O’Connell
| Born: | 1931.12.10 (Australia) | | Ordained Priest: | 1957.07.28 | | Consecrated Bishop: | 1976.03.31 | | Died: | 2013.04.27 († 81) |
| Auxiliary Bishop of Melbourne (Australia) (1976.01.24 – 2006.12.11) Titular Bishop of Saint German’s (1976.01.24 – 2013.04.27) |
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Bishop Mark O’Connell (59) Titular Bishop of Gigthi
Auxiliary Bishop of Boston (USA)
Invenimus Messiam | Born: | 1964.06.25 (Scarborough, ON, Canada) | | Ordained Priest: | 1990.06.16 | | Consecrated Bishop: | 2016.08.24 |
| Priest of Boston (USA) (1990.06.16 – 2016.06.03) Titular Bishop of Gigthi (2016.06.03 – ...) Auxiliary Bishop of Boston (USA) (2016.06.03 – ...) |
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Bishop Kevin O’Connor | Born: | 1929.05.20 (England) | | Ordained Priest: | 1954.06.12 | | Consecrated Bishop: | 1979.07.03 | | Died: | 1993.05.05 († 63) |
| Titular Bishop of Glastonbury (1979.05.28 – 1993.05.05) Auxiliary Bishop of Liverpool (England) (1979.05.28 – 1993.05.05) |
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Archbishop Martin John O’Connor |
 Bishop Laurentius Odorisi
| Born: | 1674.08 | | Ordained Priest: | 1697.12.21 | | Consecrated Bishop: | 1747.12.21 |
| Titular Bishop of Tempe (1747.12.18 – ?) Auxiliary Bishop of Sabina (Italy) (1747.12.18 – ?) |
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Bishop James Thomas O’Dowd | Born: | 1907.08.04 (USA) | | Ordained Priest: | 1932.06.04 | | Consecrated Bishop: | 1948.06.29 | | Died: | 1950.02.05 († 42) |
| Titular Bishop of Cea (1948.05.22 – 1950.02.05) Auxiliary Bishop of San Francisco (USA) (1948.05.22 – 1950.02.05) |
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 Bishop Cyprian Odyniec, S.J.
| Born: | 1749.09.13 | | Consecrated Bishop: | 1798.09.04 | | Died: | 1811.07.28 († 61) |
| Titular Bishop of Hippo Regius (Algeria) (1798.08.26 – 1811.07.28) Auxiliary Bishop of Mohilev (Belarus) (1798.11.17 – 1811.07.28) |
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Bishop Adam Odzimek Ne deficiant fides caritasque | Born: | 1944.10.07 (Radom, Poland) | | Ordained Priest: | 1969.05.31 | | Consecrated Bishop: | 1985.05.12 | | Died: | 2022.03.13 (Radom, Poland † 77) |
| Auxiliary Bishop of Sandomierz–Radom (Poland) (1985.04.03 – 1992.03.25) Titular Bishop of Tadamata (1985.04.03 – 2022.03.13) Auxiliary Bishop of Radom (Poland) (1992.03.25 – 2019.10.07) |
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Bishop Ferdinand Oesterhoff, O. Cist. | Born: | 1672.10.30 (Germany) | | Ordained Priest: | 1699.04.04 | | Consecrated Bishop: | 1724.01.22 | | Died: | 1748.10.22 († 75) |
| Auxiliary Bishop of Münster (Germany) (1723.12.20 – 1746.01.20) Titular Bishop of Agathonice (1723.12.20 – 1748.10.22) |
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Saint Bishop Isidore of Antioch (Siridion) | Died: | IV century? (Zaragoza, Spain) |
| Bishop. |
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 Saint Bishop Stephen of Antioch
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Saint Bishop Crescente of Byzantium |
Bishop Nicolò Offeri Apostolic Administrator of Boiano (Italy) (1413.03.13 – 1423) |
Saint Bishop Suitbert of Kaiserswerth | Born: | (Northumbria, England) | | Consecrated Bishop: | 693 | | Died: | 713.03.01 (Kaiserswerth, Germany) |
| Bishop. |
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Saint Bishop Mainchín of Limerick (Munchin) |
Saint Bishop Ronan of Locronan |
 Saint Bishop Mawgan of Man
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 Saint Bishop Heliodorus of Mesopotamia
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 Saint Bishop Moloc of Mortlach (Luan)
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Saint Bishop Stephen of Nicaea | Born: | (Nicaea, Turkiye) | | Died: | I century 🩸 (Reggio Calabria, Italy) |
| Bishop. Martyr. |
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Saint Bishop Quinctilis of Nicomedia | Died: | III century? 🩸 (Nicomedia, Turkiye) |
| Bishop. Martyr. |
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 Saint Bishop Trifillius of Nicosia
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 Saint Bishop Marculus of Numidia
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 Saint Bishop Clement of Ohrid
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Saint Bishop Sacerdos of Saguntum (Serdon) | Died: | 560? (Murviedro, Spain) |
| Bishop. |
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Saint Bishop Polycarp of Smyrna |
Saint Bishop Felix of Spello | Died: | 300? 🩸 (Spello, Italy) |
| Bishop. Martyr. |
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 Saint Bishop Paphnutius of Thebes
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Saint Bishop Spyridon of Trimitonte (Spiridione) | Born: | 270? | | Died: | 348.12.12 (Cyprus † 77) |
| Bishop. |
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 Saint Bishop Benignus of Utrecht
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Saint Bishop Ninian of Whithorn |
Archbishop Edwin Vincent O’Hara Sinite parvulos venire | Born: | 1881.09.06 (Lanesboro, MN, USA) | | Ordained Priest: | 1905.06.09 | | Consecrated Bishop: | 1930.10.28 | | Died: | 1956.09.11 († 75) |
| Bishop of Great Falls (USA) (1930.08.06 – 1939.04.15) Bishop of Kansas City (USA) (1939.04.15 – 1956.07.02) Archbishop ad personam (1954.06.29 – 1956.09.11) Bishop of Kansas City–Saint Joseph (USA) (1956.07.02 – 1956.09.11) |
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 Bishop John Joseph O’Hara (77)
Titular Bishop of Ath Truim
Auxiliary Bishop emeritus of New York (USA)
Jesus I trust in you | Born: | 1946.02.07 (Jersey City, NJ, USA) | | Ordained Priest: | 1984.12.01 | | Consecrated Bishop: | 2014.08.04 |
| Priest of New York (USA) (1984.12.01 – 2014.06.14) Auxiliary Bishop of New York (USA) (2014.06.14 – 2021.04.07) Titular Bishop of Ath Truim (2014.06.14 – ...) |
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Bishop Arkadiusz Okroj (56) Titular Bishop of Cufruta
Auxiliary Bishop of Pelplin (Poland)
Euntes evangelium prædicate | Born: | 1967.05.27 (Kartuzy, Poland) | | Ordained Priest: | 1992.06.06 | | Consecrated Bishop: | 2019.03.02 |
| Priest of Pelplin (Poland) (1992.06.06 – 2019.02.12) Titular Bishop of Cufruta (2019.02.12 – ...) Auxiliary Bishop of Pelplin (Poland) (2019.02.12 – ...) |
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