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It is a byproduct of historic migration of settlers to the state from the Southeastern United States such as Arkansas, Tennessee and Texas with its' "country-western" sound in the late 19th century. There's a known Native American influence in the dialect by the state's over 50 federally recognized tribal groups, some of them maintained or revived its' own native languages to this day.
Twang
The Okie Accent is largely characterized by a twang which is the pronunciation of a word with elongated vowels and adding of extra syllables to increase the length of the sound of the words which serve to create a drawl. i.e., the word "get" would be stretched out in a complex but subtle pronunciation of "gee-ut-ah", someone unfamiliar with the dialect or not listening closely would hear simply "git".
Lexicon
Some words used in rural Oklahoma but not in many other American English dialects (or with different meanings) are:
tump or tip over a container to empty the contents, (tah-uum-pah)
wallered-out or , increase of a circumference (wah'-lurd-ah a-yo-tah)