Why We Say O.K. And Have For A Long Time

I have always been a fan of the knowing the origins of cultural sayings and phrases that we uses everyday, especially the odd ones.  And when it comes to abbreviations and catch phrases there isn't a rival for the number of places, countries and people who say O.K. and you know exactly what they mean. O.K. is truly a universal way to say that everything is as it should be...but where did it come from?  And better yet, what has made it last for hundreds of years?  This video, I think, explains it nicely.