Politically, Oklahoma was very Southern after statehood.  It adopted Jim Crow laws against African Americans much like those throughout the South.  It had very difficult race relations culminating in the Tulsa Race Riots.
http://www.montgomerycollege.edu/...
Plus, Oklahoma was a Democratic bastion prior to the 1960s.

Yes, the agricultural patterns are similar to other Great Plains states.  Many Sooners came from Kansas and further north.  But African American came from the Deep South and, before them, the Five Civilized Tribes also made a journey from the Deep South.