"Look it up in the latest dictionary," suggested the President. And here is part of what was found: O.K.—a humorous or ignorant spelling of what should be "okeh," from the Choctaw language meaning, "It is so" ...1918, The Clothier and Furnisher, reprint edition, volume 93, page 87:
… how deeply the progressive element in the Democratic party throughout the country might misunderstand and even resent his putting his "okeh" on the candidacy of the Illinois leader for the senatorship, …1921, Joseph P. Tumulty, “Chapter XIV, Wilson and the Old Guard”, in Woodrow Wilson as I know Him, Reprint edition, Echo Library, published 2006, →ISBN, page 67:
1 more example
"Well, since Ron ain't goin, why don't you come on and go along with us two, Susie?" / "Okeh," Susie grinned. "Why not?"1967, James Jones, Go to the Widowmaker, Reprint edition (fiction), Random House, published 1979, →ISBN:
okeh
verb
Etymology Sometimes said to be from Choctaw oke, okeh (“it is so”). Probably simply a variant of OK.
"Once the public knows that all 'good' securities must be 'okehed' by a state official of the state wherein they are offered to the public, and the okeh verified by a local banker, …1919, Journal of the American Bankers Association: Volume 11, Reprint edition, University of Minnesota, published 2011, page 631: