It varies depending on each individual's language-using habit as English and Korean can't be put into one-to-one correspondence. For that, we say "큰 일 났다", "으악", "헉" or "헙", and the one Mytch has mentioned. Girls instinctively shout out "엄마야"(엄마=mom) when they are very surprised at something like a fierce dog's appearance, they call their mom as English speakers call their god.

"맙소사" '아이고' is like sighing. ^^

"Oh My God" Most Koreans (younger to mid generations) are quite familiar with the English expression and it has managed to permeate into the Korean vernacular a bit. A comparable Korean expression might be 아이고.