What is the real origin of “OK”?

OK is ubiquitous today. It has even been adopted by other languages. I think it is because of how useful it is. It can mean “I agree”, “I understand”, “I like it” or any number of other things. With such a popular word, of course people have come up with their own explanations of where it came from. But the one suggested in the video in this post I think is the most plausible.