Oh dank is most commonly used to replace a variety of expressions such as oh shit, oh crap, omg and wow. Or, you can over use the word and say it at the beginning of each sentence that comes out of your herpes infested mouth. :]
Guy : Guess what
Girl : What
Guy : I just found out that the girl I've been dating for the past 8 months is actually my cousin
Girl: OH DANK !