(Originating from the streets of Lagos, Nigeria) An expression of surprise, shock or astonishment often caused by something unexpected or mind blowing. It can be used as an adjective in some cases.
Mad oh is very dynamic hence, it is also a valid /sarcastic reply to any statement whatsoever. In cases where you are tired of a conversation, mad oh will do a lot of good because of its conversation terminating abilities.
You got A in that test? Mad Oh!
This movie is gonna be Mad Oh!
Boy: I think I'm in love with you
Girl: Mad Oh!