| Oakeshott of Seagrove Bay (Barony) |
Matthew Alan Oakeshott - Mon 1st May 2000
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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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| 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) |
Murrough O'Brien - Wed 21st Oct 1654
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3 July 1855 |
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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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| Ockingham (Barony) |
George - Wed 6th Apr 1689
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28 October 1708 |
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| O'Donnell (Barony) |
Augustine Thomas O'Donnell - Tue 10th Jan 2012
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- |
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| of Offaly (Earldom) |
James Fitzgerald - Thu 19th Mar 1761
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| Offord of Garvel (Barony) |
Malcolm Ian Offord - Wed 13th Oct 2021
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- |
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| Ogilvy of Airlie (Lordship) |
James Ogilvy - Thu 28th Apr 1491
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- |
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| Ogilvy of Cullen (Lordship) |
James Ogilvy - Tue 24th Jun 1698
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| Ogilvy of Deskford and Cullen (Lordship) |
James Ogilvy - Fri 24th Jun 1701
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| of Ogle (Earldom) |
William Cavendish - Mon 16th Mar 1665
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26 July 1691 |
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| Ogle (Viscountcy) |
William Ogle - Sat 23rd Dec 1645
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| Ogmore (Barony) |
David Rees Rees-Williams - Mon 10th Jul 1950
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