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 |
|---|---|---|---|---|
| Harry P. O`Neill | 1949–1953 | Pennsylvania | Democratic | 1889–1953 |
| John O`Neill | 1863–1865 | Ohio | Democratic | 1822–1905 |
| John Joseph O`Neill | 1883–1889 1891–1893 1894–1895 | Missouri | Democratic | 1846–1898 |
| Tip O`Neill | 1953–1987 | Massachusetts | Democratic | 1912–1994 |
| Daniel O`Reilly | 1879–1881 | New York | Independent Democrat | 1838–1911 |
| George F. O`Shaunessy | 1911–1919 | Rhode Island | Democratic | 1868–1934 |
| Eugene D. O`Sullivan | 1949–1951 | Nebraska | Democratic | 1883–1968 |
| Patrick B. O`Sullivan | 1923–1925 | Connecticut | Democratic | 1887–1978 |
| Donald L. O`Toole | 1937–1953 | New York | Democratic | 1902–1964 |
| Mary Rose Oakar | 1977–1993 | Ohio | Democratic | 1940– |
| P. Davis Oakey | 1915–1917 | Connecticut | Republican | 1861–1920 |
| Thomas Jackson Oakley | 1813–1815 | New York | Federalist | 1783–1857 |
| Thomas Jackson Oakley | 1827–1828 | New York | Democratic | 1783–1857 |
| Charles G. Oakman | 1953–1955 | Michigan | Republican | 1903–1973 |