An "OK time" is a time when someone is able to speak with you. They're not busy, in a bad mood, on their way to somewhere, etc. Use this phrase at the beginning of a conversation to make sure that the other person will be able to speak with you:
A: Hey, is this an OK time?
B: Uh... I'm actually in the middle of something. Can I come by in, like, 10 minutes?
You can also say "a good time":
Did I catch you at a good time?