Oklahoma constitution
Article III: suffrage
Section 3

The Legislature may enact laws providing for a mandatory primary system which shall provide for the nomination of all candidates in all elections for federal, state, county and municipal offices, for all political parties, except for the office of Presidential Elector, the candidates for which shall be nominated by the recognized political parties at their conventions. The Legislature also shall enact laws providing that citizens may, by petition, place on the ballot the names of independent, nonpartisan candidates for office, including the office of Presidential Elector.