Writers of historical fiction (stories, books, or screen plays) who use "okay" or "OK" in stories, etc., that take place somewhere besides the U.S. and sometime before the 1830s.
The OED surmises that "OK" was first used in the U.S. in the 1830s, and the oldest written reference to it comes from the Presidential election of 1840, when it came to be used to refer to President Martin Van Buren, whose nickname was "Old Kinderhook" from his birthplace in the state of New York. According to the OED, during that election campaign, supporters of Van Buren formed what was called "the OK Club."