By God/My god
By God and my god can be used as an intensifier or an interjection. Both can be used to express surprise, but by god is also used to say "certainly" or "really" in a dramatic way.
My God/by god! That guy has got three legs— ¡Díos mío! Ese tío tiene tres piernas
It was tough but, by God, it was fun!— Fue duro pero, ¡joder, fue divertido!