My English teacher keeps telling us to use okay instead of ok, and that you should always spell out any number under 21. So, twenty and below. But, really, it doesn't make too much of difference, he also has talked about how most people nowadays do it both ways, but in traditional writing and essays and stuff, it's always better to use the more formal way. I'm pretty sure that last bit is a run-on sentence, but oh well. Also, this is amazing, good job!
-Observing Anon