You can still find people arguing online that OK derives from Choctaw or from Andrew Jackson’s bad spelling, but Metcalf duly relies upon the dust-settling scholarship of Allen Walker Read, whose extensive reading in 19th-century newspapers established that the first use of OK in print, in The Boston Morning Post of March 23, 1839, was a joke: “o.k. — all correct.”
Such misspelling-based abbreviations were a fad. An earlier effort, “o.w.,” for “oll wright,” failed to catch on, whereas OK has gone globally viral.
Glad to see the great minds of the English language are dedicating themselves to important pursuits.