They have slightly different meanings, at least as I understand them. "It's okay by me" means something like, "It's okay with me, I will let it pass. I would normally say this if someone asks my permission to do something. - "Do you mind if we change our plans for tonight?" - "It's okay by me" "It's okay for me." might mean something like, "It's okay for me, but what about the other person?" - "Why do you stay up so late; don't you get tired?" "It's okay for me" (another way to say this is, "It works for me." These are very subtle differences and most of the time you can use all three interchangeably and be understood.

All could be used when someone suggests that you join them in a course of action, such as "Could you meet me at 5:00 pm?" "It's okay with me" or "It's okay by me" are better used when someone asks your permission for something, such as "May I take your daughter to the party?"

Bingo :)