I recently came across this explanation on the origin of “OK.”
“From the Greek - Ola Kala meaning “all correct” or “all well.” British proctors and instructors would use the contraction “O.K” instead of writing “ola kala” when grading papers.”
No, it’s not the origin, though that has been theorized before. The most likely origins—resting on much research and actual early print uses, which is something the Greek supposed origin lacks—is explained here: