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| Representative | Party | District | Years | District home | Note |
|---|---|---|---|---|---|
| Charles O'Neill | Republican | 2nd | March 4, 1863 – March 4, 1871 | Philadelphia | |
| March 4, 1873 – November 25, 1893 | Died | ||||
| Harry P. O'Neill | Democratic | 10th | January 3, 1949 – January 3, 1953 | ||
| Alexander Ogle | Democratic-Republican | 8th | March 4, 1817 – March 4, 1819 | Somerset | |
| Andrew J. Ogle | Whig | 18th | March 4, 1849 – March 4, 1851 | ||
| Charles Ogle | Anti-Masonic | 18th | March 4, 1837 – March 4, 1841 | Someset | |
| Whig | March 4, 1841 – May 10, 1841 | Died | |||
| Marlin E. Olmsted | Republican | 14th | March 4, 1897 – March 4, 1903 | ||
| 18th | March 4, 1903 – March 4, 1913 | ||||
| Robert Orr, Jr. | Jacksonian | 16th | October 11, 1825 – March 4, 1829 | Kittanning | |
| Edwin S. Osborne | Republican | At-large | March 4, 1885 – March 4, 1889 | ||
| 12th | March 4, 1889 – March 4, 1891 | ||||
| James H. Osmer | Republican | 27th | March 4, 1879 - March 4, 1881 | ||
| Edward Overton, Jr. | Republican | 15th | March 4, 1877 – March 4, 1881 | Towanda |
Read more about this topic: List Of United States Representatives From Pennsylvania