| Professor Denis O'Brien FBA | Year of birth 1938 | Year of death 2023 | Year elected 1988 | Fellowship category UK Emeritus |
| Professor Michael O'Brien FBA | Year of birth 1948 | Year of death 2015 | Year elected 2008 | Fellowship category UK |
| Professor Michael Oakeshott FBA | Year of birth 1901 | Year of death 1990 | Year elected 1966 | Fellowship category UK |
| Sir Walter Oakeshott FBA | Year of birth 1903 | Year of death 1987 | Year elected 1971 | Fellowship category UK |
| Dr Kenneth Oakley FBA | Year of birth 1911 | Year of death 1981 | Year elected 1957 | Fellowship category UK |
| Professor David Oates FBA | Year of birth 1927 | Year of death 2004 | Year elected 1974 | Fellowship category UK |
| Dr Joan Oates FBA | Year of birth 1928 | Year of death 2023 | Year elected 2004 | Fellowship category UK |
| Mr John Oates FBA | Year of birth 1912 | Year of death 1990 | Year elected 1976 | Fellowship category UK |
| Professor Dr Heiko Oberman FBA | Year of birth 1930 | Year of death 2001 | Year elected 1988 | Fellowship category Corresponding |
| Sir Dimitri Obolensky FBA | Year of birth 1918 | Year of death 2001 | Year elected 1974 | Fellowship category UK |
| Professor Hilary Offler FBA | Year of birth 1913 | Year of death 1991 | Year elected 1974 | Fellowship category UK |
| Professor Robert Ogilvie FBA | Year of birth 1932 | Year of death 1981 | Year elected 1972 | Fellowship category UK |
| Dr Sarwat Okasha FBA | Year of birth 1921 | Year of death 2012 | Year elected 1975 | Fellowship category Corresponding |
| Professor Alexei Pawlowitsch Okladnikov FBA | Year of birth 1908 | Year of death 1981 | Year elected 1973 | Fellowship category Corresponding |
| Professor Olderogge FBA | Year of birth 1900 | Year of death 2000 | Year elected 1975 | Fellowship category Corresponding |
| Professor Roland Oliver FBA | Year of birth 1923 | Year of death 2014 | Year elected 1993 | Fellowship category UK Emeritus |