Lyndon, KS, is halfway but Topeka's a better choice.
Tulsa.
West Plains, Missouri is halfway between Tulsa and Clarksville
The halfway point between Gulfport, Mississippi and Tulsa, Oklahoma is Varner, Arkansas. There is approximately 564 miles between Tulsa and Gulfport.
The halfway point between Tulsa, Oklahoma, and Vicksburg, Mississippi, is Conway, Arkansas, if you're going via Fort Smith, Little Rock, and Pine Bluff.
The halfway point between Tulsa, OK and Fort Campbell, KY falls in or around Mountain View, MO.
Edwardsville, IL
Des Monien Iowa
Galena, OH
Memphis, TN
Urbana, Illinois.
Terre Haute, Indiana
Somewhere near Tulsa, Oklahoma.