I found this on a website that people look for places by coordinates.A few people posted on their forum that they did indeed find Post Oak Cemetary.They post the coordinates and this small note.There is also history notes of this area posted on the Choctaw Nation website.
"Gilbert Dukes, Choctaw chief 1900-1902, is buried at Post Oak Cemetery, south of the Kiamichi River and about twelve miles southeast of Talihina. Dukes played an important part in the selection of the site of the first Choctaw Indian Hospital a few miles northwest of Talihina. Genology accounts state that this cemetery was near Post Oak Indian Presbyterian Church; however, I have not located any ruins or building that would be this church. This is a small cemetery with scattered trees and mountains in the background making it a quiet and peaceful place to visit. There are only a few marked graves with the most recent ones in the the early 1920's."