List of former members of the United States House of Representatives (O) Number of years/terms representative/delegate has served Number of years/terms representative/delegate has served

| Representative | Years | State_ | Party | Lifespan |
|---|---|---|---|---|
| Mordecai Oliver | 1855–1857 | Missouri | Oppositionist | 1819–1898 |
| S. Addison Oliver | 1875–1879 | Iowa | Republican | 1833–1912 |
| William Bacon Oliver | 1915–1937 | Alabama | Democratic | 1867–1948 |
| William M. Oliver | 1841–1843 | New York | Democratic | 1792–1863 |
| Marlin Edgar Olmsted | 1897–1913 | Pennsylvania | Republican | 1847–1913 |
| Richard Olney II | 1915–1921 | Massachusetts | Democratic | 1871–1939 |
| Archibald E. Olpp | 1921–1923 | New Jersey | Republican | 1882–1949 |
| Arnold Olsen | 1961–1971 | Montana | Democratic | 1916–1990 |
| Alec G. Olson | 1963–1967 | Minnesota | Democratic-Farmer-Labor | 1930– |
| Stephen Ormsby | 1811–1813 1813–1817 | Kentucky | Democratic-Republican | 1759–1844 |
| Alexander D. Orr | 1792–1795 | Kentucky | Anti-Administration | 1761–1835 |
| Alexander D. Orr | 1795–1797 | Kentucky | Democratic-Republican | 1761–1835 |
| Benjamin Orr | 1817–1819 | Massachusetts | Federalist | 1772–1828 |
| Jackson Orr | 1871–1875 | Iowa | Republican | 1832–1926 |
| James Lawrence Orr | 1849–1859 | South Carolina | Democratic | 1822–1873 |