| Oakeshott of Seagrove Bay (Barony) |
Matthew Alan Oakeshott - Mon 1st May 2000
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- |
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| Oakley (Barony) |
George Cadogan - Sat 10th Sep 1831
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- |
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| Oaksey (Barony) |
Geoffrey Lawrence - Mon 13th Jan 1947
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- |
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| Oakshott (Barony) |
Hendrie Dudley Oakshott - Fri 21st Aug 1964
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1 February 1975 |
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| Oates (Barony) |
Jonathan Oates - Mon 5th Oct 2015
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| O'Brien (Barony) |
Peter O'Brien - Sat 16th Jun 1900
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7 September 1914 |
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| O'Brien of Lothbury (Barony) |
Leslie Kenneth O'Brien - Wed 14th Mar 1973
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24 November 1995 |
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| O'Cathain (Barony) |
Detta O'Cathain - Fri 21st Jun 1991
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23 April 2021 |
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| Ockham (Viscountcy) |
William King - Sat 30th Jun 1838
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31 January 2018 |
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| O'Donnell (Barony) |
Augustine Thomas O'Donnell - Tue 10th Jan 2012
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- |
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| Offord of Garvel (Barony) |
Malcolm Ian Offord - Wed 13th Oct 2021
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- |
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| Ogmore (Barony) |
David Rees Rees-Williams - Mon 10th Jul 1950
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- |
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| O'Grady of Upper Holloway (Barony) |
Frances Lorraine Maria O'Grady - Fri 9th Dec 2022
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- |
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| O'Hagan (Barony) |
Thomas O'Hagan - Tue 14th Jun 1870
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- |
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| Oliver of Aylmerton (Barony) |
Peter Raymond Oliver - Fri 31st Jan 1986
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17 October 2007 |
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| Olivier (Barony) |
Sydney Haldane Olivier - Sat 9th Feb 1924
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15 February 1943 |
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| Olivier (Barony) |
Laurence Kerr Olivier - Fri 5th Mar 1971
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11 July 1989 |
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| O'Loan (Barony) |
Nuala Patricia O'Loan - Fri 11th Sep 2009
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- |
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| O'Neill (Barony) |
William O'Neill - Sat 18th Apr 1868
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- |
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| O'Neill of Bengarve (Barony) |
Onora Sylvia O'Neill - Thu 25th Feb 1999
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