(This answer is from the viewpoint of a northeastern US English speaker. Other dialects may have different answers.)
In this case only with is really idiomatic. "OK with you" means you find it good, or you don't have a problem with it. "OK to you" means something more like "behaving acceptably towards you".
But the confusion might come from the fact that there are many constructions where you would say "OK to you". For example, "Does that look OK to you?" But that's part of the construction "to look X to Y", meaning "to appear to Y have an X nature".
Informally, I've also heard people say "Is that OK by you?" or "Is that OK for you?" so you have a wide range of choices!