Can informed sources please also comment on whether US President Martin Van Buren is the source of 'Okay'? I'm aware of received wisdom regarding this which states that via his nickname of Old Kinderhook (his hometown) we pulled into existence our OK.

Could a knowledgable responder confirm or dispel this? Thanks!

If no one is able to help you here, perhaps the folks at /r/etymology or /r/linguistics can.