Why would a frontier state (at that time) give a socialist 16% of the vote? Because Teddy Roosevelt (Bull Moose) couldn't get on the ballot, so most of his supporters voted for Debs as a protest.
Not so; Debs took 8% there in 1908 (when he was polling just under 3% nationally) and Benson took 15% there in 1916. Note that the Socialists were strongest in the likes of Little Dixie rather than in more Republican areas.
The reason for the sharp Socialist decline in Oklahoma after the First World War is because their organisation was cynically destroyed by the state's political establishment during the war (of course this goes for the Socialist Party across the U.S in certain respects; but the repression was worse in Oklahoma than elsewhere).
