Okay bye is another form of asking " are you joking me?". It can also mean "whatever" or "I can't believe he/she said that!". You can also use it when you see something that is not okay, disturbing, or really funny. It is commonly used in the female population.
Boy: " You look like Air Bud"
Girl: OKAYBYE! No I don't
Girl 1: " Did you see that girls camel toe?!"
Girl 2: "Yeah and i was like OKAY BYE!"