| O'Grady (Barony) |
Standish O'Grady - Thu 6th Jan 1831
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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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- |
- |
| O'Hagan (Barony) |
Thomas O'Hagan - Tue 14th Jun 1870
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- |
- |
| Oliver of Aylmerton (Barony) |
Peter Raymond Oliver - Fri 31st Jan 1986
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17 October 2007 |
- |
| Olivier (Barony) |
Sydney Haldane Olivier - Sat 9th Feb 1924
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15 February 1943 |
- |
| Olivier (Barony) |
Laurence Kerr Olivier - Fri 5th Mar 1971
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11 July 1989 |
- |
| O'Loan (Barony) |
Nuala Patricia O'Loan - Fri 11th Sep 2009
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- |
- |
| O'Neill (Earldom) |
Charles Henry O'Neill - Thu 28th Aug 1800
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- |
- |
| O'Neill (Barony) |
John O'Neill - Fri 25th Oct 1793
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- |
- |
| O'Neill (Barony) |
William O'Neill - Sat 18th Apr 1868
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- |
- |
| O'Neill (Viscountcy) |
John O'Neill - Sat 3rd Oct 1795
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- |
- |
| O'Neill of Bengarve (Barony) |
Onora Sylvia O'Neill - Thu 25th Feb 1999
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- |
- |
| O'Neill of Bexley (Barony) |
Teresa Ann Jude O'Neill - Mon 7th Nov 2022
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- |
- |
| O'Neill of Clackmannan (Barony) |
Martin John O'Neill - Tue 14th Jun 2005
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26 August 2020 |
- |
| O'Neill of Gatley (Barony) |
Terence James O'Neill - Thu 28th May 2015
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- |
- |
| O'Neill of the Maine (Barony) |
Terence Marne O'Neill - Fri 23rd Jan 1970
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12 June 1990 |
- |
| Ongley (Barony) |
Robert Henley Ongley - Tue 30th Jul 1776
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- |
- |
| of Onslow (Earldom) |
George Onslow - Fri 19th Jun 1801
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- |
- |
| Onslow (Barony) |
Richard Onslow - Fri 19th Jun 1716
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- |
- |
| Onslow of Woking (Barony) |
Cranley Gordon Douglas Onslow - Fri 31st Oct 1997
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13 March 2001 |
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