O
| Senator | Years | State | Party |
|---|---|---|---|
| Herbert R. O'Conor | 1947–1953 | Maryland | Democratic |
| W. Lee O'Daniel | 1941–1949 | Texas | Democratic |
| James Aloysius O'Gorman | 1911–1917 | New York | Democratic |
| Joseph C. O'Mahoney | 1934–1953, 1954–1961 |
Wyoming | Democratic |
| Barack Obama | 2005–2008 | Illinois | Democratic |
| Tasker Oddie | 1921–1933 | Nevada | Republican |
| Aaron Ogden | 1801–1803 | New Jersey | Federalist |
| Richard James Oglesby | 1873–1879 | Illinois | Republican |
| Simeon Olcott | 1801–1805 | New Hampshire | Federalist |
| George T. Oliver | 1909–1917 | Pennsylvania | Republican |
| Thomas W. Osborn | 1868–1873 | Florida | Republican |
| Harrison Gray Otis | 1817–1822 | Massachusetts | Federalist |
| Lee Slater Overman | 1903–1930 | North Carolina | Democratic |
| John H. Overton | 1933–1948 | Louisiana | Democratic |
| Robert L. Owen | 1907–1925 | Oklahoma | Democratic |
Read more about this topic: List Of Former United States Senators