An exclamation of surprise, excitement, sadness, or fear. Pretty much the wimpy way of saying "oh my god." usually used by strong christians or when in a group of christians and you just want to be safe and not offend anyone.
Person 1: I just saw a dead deer on the side of the road!
Person 2: Oh my gosh, that's so sad!
Oh my gosh! I'm so excited to go to Emma's birthday party!
Oh my gosh, you scared me! I didn't see you standing there.
Oh my gosh, what if my parents find out about me failing my math quiz?? Oh no!