How to say,"OMG" or express shock!
Express shock or say oh my god in korean, for example ibam shocked at my korean friend telling me something surprising

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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

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Oh my god - 세상에!! 어머나!!! 헐 말도 안돼!!

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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!

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맙소사

세상에

말도 안 돼 = it can't be