First off, what is today the state of Oklahoma is the result of
three "leftover" pieces of territory. The eastern part of the
state was reserved for the "Civilized" (aka farming) tribes
pushed out of the American Southeast. The western half was later
divied up to other tribes (eg: the Osages) as they got pushed
out of their territories. Generally they weren't force-marched
into their territories by the army though, so the tribal element
in the western reservations wasn't nearly as strong as in the
east. The "panhandle" was part of Texas, but was too far North
to fit in the slave state/free state compromise that allowed
Texas to join the Union, and too far south to fit in a similar
compromise that brought in Kansas. As for the Indian Territory
(eastern half of Oklahoma) itself, it was originally supposed to
be land set aside for the tribe's use. Each tribe had its own
specific territory which it was supposed to govern. The problem
was white folk didn't tend to respect tribal governments, so
effectively the Ind