OK was first used on March 23 in the morning edition of the "Boston Morning Post" in the distant 1839. In a short humorous piece, the activities of an organization that had declared itself against the use of bells at the table to call service personnel are reported.
In one passage, the phrase " o.k.—all correct" appears, from which it is understood that OK stands for all correct (completely correct). The mistake is deliberate, since at that time (as now, by the way) intentional errors were observed in written speech for more color and to achieve implausibility in the meaning, satirical emphasis.
However, the abbreviation became nationally known in 1940 with the presidential election. OK is an abbreviation of the nickname of the Democratic candidate Martin Van Buren - Old Kinderhook (he was a native of the town of Kinderhook). His opponent began to use it mockingly, but Van Buren's team quickly turned the abbreviation to his advantage, linking it to the phrase used by the "Boston Post" o.k. - precisely in the sense of positive, constructive and credible.
And this contributed to a positive image of Van Buren.