맙소사 - This expression is commonly used in written form and not really in conversational Korean. For a slang 'shocking' expression, I would usually hear them say: 헐 (heol) for "What the...." 엄마야 (eom ma ya) for OMG or oops

Oh my god - 세상에!! 어머나!!! 헐 말도 안돼!!

Other expressions in conversational Korean: 대박! (daebak) - cool/ awesome 우와! (uwa) - wow 진 ㅈ자 요? (jinjjayo) - For real? 그래요? (geuraeyo) - Is that so? 정말요? (jeongmalyo) - Really? 아이고! (aigo) - *sigh*/ whew/ oops 아싸! (assa) - Yay!/ Hurrah!

맙소사 세상에 말도 안 돼 = it can't be