"Oh No Bro" is a phrase derived from the time that an absolutely goofy goober whose name shall not be disclosed getting their ass handed to them in Whispy Woods as Pink Luigi. It means that you just got fucking pwned in Whispy fucking Woods.
When you fail at something in life (eg. failing an exam or not getting a job), go home and spend an hour just standing in the shower, propping yourself up against the wall with your eyes on your feet.
Caleb knew the only thing to do after he didnt get prefect was take a failureshower.