The Leader of the Opposition in Ontario is usually leader of the largest party in the Ontario legislature which is not the government. The current official opposition is formed by the Ontario Progressive Conservative Party; Patrick Brown is the current Leader of the Opposition.

Ontario's first Leader of the Opposition was Edward Blake of the Ontario Liberal Party who held the position from 1869 until 1871 when he became Premier of Ontario (Archibald McKellar had previously led the Liberal Party in the legislature for two years, but was not formally recognized as opposition leader).

• Archibald McKellar (Liberal) 1867-1869 was not formally recognized as opposition leader, but led the Liberal Party in the legislature.

Leaders of the Opposition

Liberal Leader

• Edward Blake (December 1869-December 1871)

Conservative Leaders

• Matthew Crooks Cameron (December 1871-1878)
• William Ralph Meredith (October 1878-October 1894)
• George Marter (October 1894-April 1896)
• James Whitney (April 1896-January 1905)

Liberal Leaders

• George William Ross (February 1905-January 1907)
• George Graham (January 1907-August 1907)
• Alexander Grant MacKay (August 1907-1911)
• Newton Wesley Rowell (December 1911-1917)
• William Proudfoot (February 1918-October 1919)
• Hartley Dewart (1919-October 1921)
• Wellington Hay (March 1922-June 1923)
• William Sinclair (August 1923-June 1934)

Conservative Leader

• George Henry (July 1935-December 1938)

Conservative/Progressive Conservative Leader

• George Drew (1939-1943)

Co-operative Commonwealth Federation (Ontario Section) Leader

• Ted Jolliffe (August 1943-June 1945)

Liberal Leader

• Farquhar Oliver (July 1945-June 1948)

Co-operative Commonwealth Federation (Ontario Section) Leader

• Ted Joliffe (2nd time) (July 1948-November 1951)

Liberal Leaders

• Farquhar Oliver (2nd time) (1951-April 1958)
• John Wintermeyer (April 1958-August 1963)
• Farquhar Oliver (3rd time) (October 1963-September 1964)
• Andy Thompson (September 1964-November 1966)
• Robert Nixon (February 1967-September 18, 1975)

New Democratic Leader

• Stephen Lewis (October 28, 1975-April 29, 1977)

Liberal Leaders

• Stuart Smith (June 1977-September 1981)
• Robert Nixon (2nd time) (January 25, 1982-February 21, 1982)
• David Peterson (February 1982-June 1985)

New Democratic Leader

• Bob Rae (1987-1990)

Liberal Leaders

• Robert Nixon (November 20, 1990-July 31, 1991
• Murray Elston (1991-1991)
• Jim Bradley (1991-1992)
• Lyn McLeod (1992-1996)
• Dalton McGuinty (1996-2003)

Progressive Conservative Leaders

• Ernie Eves (2003-2004)
• Bob Runciman (2004-2005)
• John Tory (2005-2007)
• Bob Runciman (2nd time) (2007-2009)
• Tim Hudak (2009-2014)
• Jim Wilson (2014-2015)
• Patrick Brown (2015-Present)