| Title | Date of creation | Surname | Current status | Notes | | O'Brien of Ardtona[1] | 1916 | O'Brien | extinct 1916 | first Baronet created Baron Shandon in 1918 |
| O'Brien of Kilfenora[2] | 1891 | O'Brien | extinct 1914 | first Baronet created Baron O'Brien in 1900 |
| O'Brien of Merrion Square and Borris-in-Ossory[3] | 1849 | O'Brien | dormant | Lord Mayor of Dublin; sixth Baronet died 1982 |
| O'Loghlen of Drumcanora[4] | 1838 | O'Loghlen | extant |
| O'Malley of Rosehill[5] | 1804 | O'Malley | extinct 1892 |
| O'Connell of Lakeview[6] | 1869 | O'Connell | extant |
| O'Neill of Cleggan[7] | 1929 | O'Neill | extant | first Baronet created Baron Rathcavan in 1953 |
| date=September 2012}} | 1815 | Oakes | extinct 1927 | first Baronet had already been created a Baronet in 1813, which creation became extinct in 1822 |
| Oakes of Nassau[8] | 1939 | Oakes | extant |
| date=September 2012}} | 1813 | Oakes | extinct 1822 | first Baronet obtained a new patent in 1815, which creation became extinct in 1927 |
| Oakshott[9] | 1959 | Oakshott | extant | first Baronet created a life peer as Baron Oakshott in 1964, which title became extinct in 1975 |
| date=September 2012}} | 1823 | Ochterlony | extinct 1964 | had already been created Baronet of Pitforthy in 1816, which title became extinct in 1825 |
| Ochterlony of Pitforthy[10] | 1816 | Ochterlony | extinct 1825 | also created Baronet of Ochterlony in 1823, which title became extinct in 1964 |
| Ogle of Worthy[11] | 1816 | Ogle | extinct 1940 |
| Ohlson of Scarborough[12] | 1920 | Ohlson | extant |
| Oppenheimer of Stoke Poges[13] | 1921 | Oppenheimer | extant |
| Ormsby of Cloghans[14] | 1812 | Ormsby | extinct 1833 |
| Orr-Ewing of Ballikinran[15] | 1886 | Orr-Ewing | extant |
| Orr-Ewing of Hendon[16] | 1963 | Orr-Ewing | extant | first Baronet created a life peer as Baron Orr-Ewing in 1971, which title became extinct in 1999 |
| Orr-Lewis of Whitewebbs Park[12] | 1920 | Orr-Lewis | extinct 1980 |
| Osler of Norham Gardens[17] | 1911 | Osler | extinct 1919 |
| Otway of Brighthelmstone[18] | 1831 | Otway | extinct 1912 |
| Ouseley of Claremont[19] | 1808 | Ouseley | extinct 1889 |
| Outram of The Bengal[20] | 1858 | Outram | extant |
| Owen of Orielton[14] | 1813 | Owen | extinct 2002 |
1. ^{{London Gazette |issue=29483 |date=22 February 1916 |page=1946 }}
2. ^{{London Gazette |issue=26207 |date=29 September 1891 |page=5086 }}
3. ^{{London Gazette |issue=21013 |date=28 August 1849 |page=2669 }}
4. ^{{London Gazette |issue=19631 |date=3 July 1838 |page=1488 }}
5. ^{{London Gazette |issue=15712 |date=19 June 1804 |page=765 }}
6. ^{{London Gazette |issue=23544 |date=8 October 1869 |page=5446 }}
7. ^{{London Gazette |issue=33509 |date=25 June 1929 |page=4189 }}
8. ^{{London Gazette |issue=34651 |date=4 August 1939 |page=5392 }}
9. ^{{London Gazette |issue=41764 |date=14 July 1959 |page=4465 }}
10. ^{{London Gazette |issue=17082 |date=21 November 1815 |page=2328 }}
11. ^{{London Gazette |issue=17102 |date=23 January 1816 |page=133 }}
12. ^1 {{London Gazette |issue=31830 |date=19 March 1920 |page=3432 }}
13. ^{{London Gazette |issue=32257 |date=15 March 1921 |page=2092 }}
14. ^1 {{London Gazette |issue=16663 |date=31 October 1812 |page=2189 }}
15. ^{{London Gazette |issue=25564 |date=2 March 1886 |page=1027 }}
16. ^{{London Gazette |issue=43041 |date=28 June 1963 |page=5533 }}
17. ^{{London Gazette |issue=28566 |date=29 December 1911 |page=9826 }}
18. ^{{London Gazette |issue=18851 |date=16 September 1831 |page=1898 }}
19. ^{{London Gazette |issue=16185 |date=20 September 1808 |page=1303 }}
20. ^{{London Gazette |issue=22190 |date=12 October 1858 |page=4433 }}