| Player | Birth date | First year in majors | Last year in majors | Played in majors | With StL-N | With minors | Drafted |
| Bill O'Hara | August 14, 1881 | 1909 | 1910 | Yes | Yes | No | 0 |
| Tom O'Hara | July 13, 1880 | 1906 | 1907 | Yes | Yes | No | 0 |
| Dennis O'Higgins | June 15, 1913 | 0 | 0 | No | No | Yes | 0 |
| Brian O'Keefe | July 15, 1993 | 2022 | 2023 | Yes | No | Yes | 2014 |
| Patrick O'Keefe | 1930 | 0 | 0 | No | No | Yes | 0 |
| Tim O'Keefe | January 4, 1966 | 0 | 0 | No | No | Yes | 0 |
| Thomas O'Laughlin | January 3, 1917 | 0 | 0 | No | No | Yes | 0 |
| Charley O'Leary | October 15, 1875 | 1904 | 1934 | Yes | Yes | No | 0 |
| Jerry O'Leary | 1915 | 0 | 0 | No | No | Yes | 0 |
| Bob O'Neal | December 8, 1925 | 0 | 0 | No | No | Yes | 0 |
| Randy O'Neal | August 30, 1960 | 1984 | 1990 | Yes | Yes | Yes | 0 |
| Dennis O'Neill | November 22, 1866 | 1893 | 1893 | Yes | Yes | No | 0 |
| Doc O'Neill | 1917 | 0 | 0 | No | No | Yes | 0 |
| Jack O'Neill | January 10, 1873 | 1902 | 1906 | Yes | Yes | No | 0 |
| John O'Neill | 1933 | 0 | 0 | No | No | Yes | 0 |
| Mike O'Neill | September 7, 1877 | 1901 | 1907 | Yes | Yes | No | 0 |
| Mike O'Neill | February 12, 1988 | 0 | 0 | No | No | Yes | 2010 |
| Roger O'Neill | 0 | 0 | No | No | Yes | 0 | |
| Steve O'Neill | July 6, 1891 | 1911 | 1928 | Yes | Yes | No | 0 |
| Tip O'Neill | May 15, 1860 | 1883 | 1892 | Yes | Yes | No | 0 |