The Fortieth Oklahoma Legislature was a meeting of the legislative branch of the government of Oklahoma, composed of the Senate and the House of Representatives. It met in Oklahoma City from January 8, 1985, to January 6, 1987, during the term of Governor George Nigh. It was marked by the enactment of the Executive Branch Reform Act of 1986 and the establishment of the franchise tax in Oklahoma.

The president pro tempore of the Oklahoma Senate is the second-highest-ranking official of the Oklahoma Senate and the highest-ranking state senator. The Oklahoma Constitution designates the Lieutenant Governor of Oklahoma as the highest-ranking official, serving ex officio as President of the Senate, even though the lieutenant governor only votes in the case of a tie. During the lieutenant governor's absence, the president pro tempore presides over sessions. By longstanding custom, the lieutenant governor presides over sessions devoted to ceremonial purposes, while the bulk of the legislative management and political power is reserved for the president pro tempore, who is elected directly by the Oklahoma Senate.