이럴수가 or 오 이런! are same expressions as oh my god! Now let me teach you some slang expressions that are commonly used among teenagers and on the internet. 헐, 쩐다, 레알, and 에바. Those are not formal words, but sounds funny^^ I hope you enjoy studying korean. have a nice day

i just want to add to the above explanation a slang expression that is very often used, especially by girls '어머' and the meaning is something like 'oh my' and could be used instead of 'oh my god' due to a reason that it's used very very often