William John Copley was a South Australian politician. He was a member of the South Australian House of Assembly from 1884 to 1887, representing Frome, switched to the South Australian Legislative Council from 1887 to 1894 representing Northern District, before returning to the House of Assembly from 1896 to 1904, representing Yorke Peninsula. He was Leader of the Opposition for a period in 1896, and served as Commissioner for Crown Lands and Immigration (1890-1892) and Minister for Agriculture and Education (1892) under Thomas Playford II and Chief Secretary (1893) and Minister for Agriculture and Education (1892-1893) under John Downer.

The Leader of the Opposition in South Australia is the leader of the largest minority political party or coalition of parties, known as the Opposition, in the House of Assembly of the Parliament of South Australia. By convention, the leader of the opposition is a member of the House of Assembly. The leader acts as the public face of the opposition, and acts as a chief critic of the government and ultimately attempt to portray the opposition as a feasible alternate government. They are also given certain additional rights under parliamentary standing orders, such as extended time limits for speeches. Should the opposition win an election, the Leader of the Opposition will be nominated to become the Premier of South Australia.