OK actually dates back back to 1790, assuming Wikipedia is correct (and this is never a safe assumption, I know). In any event, "OK" seems to predate "okay." "Alright," on the other hand, is more recent than "all right," and thus hardcore grammarians do tend to raise their hackles about it.

While the word itself may date back that far (I really don't know if Wiki is correct or not), there are still (grammarian) rules for when it is okay to use ok - and in a work of prose would not be ok. <------I can't help it. I'm compelled to pun, even when it's very very bad. Someone shoot me now.