The only thing that experts are sure of is that “OK” is of American origin. How this word came into use no one is really sure. Several theories abound. According to some people, the Chocktaw tribe of American Indians used it to mean, “It is so”.
Others are of the opinion that a railroad clerk coined this word. It is believed that whenever someone gave Obadiah Kelly a package to be mailed, he put his initials (O.K) on it. What it meant was that the package had been inspected and paid for.