OK

Origin disputed. Wikipedia lists many possible etymologies, of which the most widely accepted is that it is an abbreviation of oll/orl korrect, a comical spelling of all correct, which first appeared in print in The Boston Morning Post on March 23, 1839, as part of a fad for similar fanciful abbreviations in the United States during the late 1830s. The expression became popular through its use in the presidential campaign of Martin Van Buren in 1840, who was nicknamed Old Kinderhook, and then slowly acquired other meanings. The Choctaw word oke, okeh (“it is so”), common in Choctaw translations of the Bible, could also explain OK's variety of affirmative definitions. Additionally, okeh was the most common etymology of okay in dictionaries until the 1960s, and linguistically predates Boston's O.K.. However, this theory suffers from the fact that the Choctaw language was relatively obscure and generally spoken (sometimes in a pidgin form) mainly with African-American slaves.

来源：wiktionary