According to
this article, no third party was able to obtain ballot access in Oklahoma with the strange exception of Americans Elect. I read on another site that third parties need to obtain a number of signatures equal to 5% of the votes cast in the most recent election to be on the ballot, which is a very high threshold, but I don't understand how Americans Elect could obtain the necessary signatures but neither the Libertarian nor Green Party were able to do so as well. Would anyone know why this might be the case?