| Senator | Lifespan | Party [1] | Prov. | Entered | Left | Appointed by | Left due to | For life? |
|---|---|---|---|---|---|---|---|---|
| James O'Brien | 1836–1903 | LC | QC | 2 January 1896 | 28 May 1903 | Bowell | Death | Y |
| Michael John O'Brien | 1851–1940 | C | ON | 1 September 1918 | 1 September 1925 | Borden | Resignation | Y |
| Frank Patrick O'Connor | 1885–1939 | C | ON | 6 December 1935 | 21 August 1939 | King | Death | Y |
| William Hunter Odell | 1811–1891 | C | NB | 23 October 1867 | 25 July 1891 | Royal proclamation | Death | Y |
| John O'Donohoe | 1824–1902 | LC | ON | 21 May 1882 | 7 December 1902 | Macdonald | Death | Y |
| Kelvin Ogilvie | 1942–present | C | NS | 27 August 2009 | 6 November 2017 | Harper | Retirement | |
| Victor Oh | 1949–present | C | ON | 25 January 2013 | 9 June 2024 | Harper | Retirement | |
| Clement O'Leary | 1916–1969 | PC | NS | 25 September 1962 | 12 June 1969 | Diefenbaker | Death | Y |
| Grattan O'Leary | 1888–1976 | PC | ON | 24 September 1962 | 7 April 1976 | Diefenbaker | Death | Y |
| Alexander Walker Ogilvie | 1829–1902 | C | QC | 24 December 1881 | 18 January 1901 | Macdonald | Resignation | Y |
| Donald Oliver | 1938–present | C | NS | 7 September 1990 | 16 November 2013 | Mulroney | Retirement | |
| Louis Auguste Olivier | 1816–1881 | C | QC | 23 October 1867 | 8 September 1873 | Royal proclamation | Resignation | Y |
| Bud Olson | 1925–2002 | C | AB | 5 April 1977 | 7 March 1996 | Trudeau, P. | Resignation | |
| Ratna Omidvar | 1949–present | NA | ON | 1 April 2016 | — | Trudeau, J. | — | |
| Gigi Osler | 1968–present | MB | 26 September 2022 | — | Trudeau, J. | — | ||
| Gerry Ottenheimer | 1934–1998 | PC | NL | 30 December 1987 | 18 January 1998 | Mulroney | Death | |
| Manuelle Oudar | 1963–present | QC | 13 February 2024 | — | Trudeau, J. | — | ||
| William Owens | 1840–1917 | C | QC | 2 January 1896 | 8 June 1917 | Bowell | Death | Y |