Sir Oliver Mowat GCMG PC QC (July 22, 1820 – April 19, 1903) was a Canadian lawyer, politician, and Ontario Liberal Party leader.

He is best known for defending successfully the constitutional rights of the provinces in the face of the centralizing tendency of the national government as represented by his longtime Conservative adversary, John A. Macdonald.

This longevity and power was due to his maneuvering to build a political base around Liberals, Catholics, trade unions, and anti-French-Canadian sentiment.

Mowat served as Provincial Secretary (1858) and Postmaster-General (1863–1864) in the pre-Confederation governments of George Brown and John Sandfield Macdonald for the Liberal Party of Canada.

[7] Mowat served as provincial member for the riding of Oxford North, about 150 km west of Toronto, for his entire term as premier.

As premier in the 1880s a series of disputes with the Dominion arose over Provincial boundaries,[8] jurisdiction over liquor licenses,[9] trade and commerce,[10] rivers and streams,[11] timber,[12] escheats,[13] and other matters.

He also served as his own Attorney-General concurrently with his service as Premier, and introduced reforms such as the secret ballot in elections,[15] and the extension of suffrage beyond property owners.

[18] His policies, particularly regarding liquor regulation and separate schools, routinely drew criticism from political conservatives, including the Orange Lodge and its associated newspaper, The Sentinel.

[20] The Judicial Committee of the Privy Council in Britain, serving as Canada's highest appeal court, repeatedly issued rulings taking the side of provincial rights.

[23] In 1884, when the federal government urged that Ontario transfer to the Teme-Augama Anishnabai indigenous people all or some of the 2,770 square miles as a reserve, for which that band's head chief, Ignace Tonené, had campaigned with the federal authorities for many years, Mowat blocked the land transfer, primarily concerned about the value of the red and white pine lumber at the location.

It was thought that the combination of a French Canadian (Laurier) and the prestige of Oliver Mowat in Ontario would be a winning ticket for the Liberal party.

The two former Kingston law partners, Macdonald as prime minister in Ottawa and Mowat as premier in Toronto, led their respective governments during the same era for a total of 14 years.

After his death, Wilfrid Laurier placed Mowat's policy of sectarian tolerance second in historical importance only to his role in giving Confederation its character as a federal compact.

Mowat was portrayed by David Onley (the 28th Lieutenant-Governor of Ontario) in the Canadian TV series Murdoch Mysteries in 2013 in the episode "The Ghost of Queen's Park.