| Richard James "Uncle Dick" Oglesby | July 25, 1824 - April 24, 1899 |
| John Morrison Oliver | September 6, 1828 - March 30, 1872 |
| "Samuel" Emerson Opdyke | January 7, 1830 - April 25, 1884 |
| Edward Otho Cresap Ord | October 18, 1818 - July 22, 1883 |
| William Ward Orme | February 17, 1832 - September 13, 1866 |
| Thomas Ogden Osborn | August 11, 1832 - March 27, 1904 |
| Peter Joseph Osterhaus | January 4, 1823 - January 2, 1917 |
| Joshua Thomas Owen | March 29, 1821 - November 7, 1887 |
| Edward Asbury O'Neal | September 20, 1818 - November 7, 1890 |
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