Here are some popular expressions used by Koreans when they are surprised or express exclamations. 

1. 엄마야! [eom-ma-ya!] = Oh My God! / Oops!
2. 대박 [dae-bak] = Super cool! / Awesome!
3. 우와 [u-wa] = Wow 
4. 진짜요? [jin-jja-yo?] = For real?
5. 그래요? [geu-rae-yo?] = Is that so?
6. 정말요? [jeong-mal-yo?] = Really?
7. 아이고! [a-i-go!] = *sigh* / Whew! / Oops!
8. 아싸 [a-ssa] = Yay! / Hurrah!
9. 네? [ne?] = Yes?
10. 어? [eo?] = Huh?
11. 아/아야 [a/a-ya] = Ouch!
12. 헐 [heol] = What the...
13. 헉! [heok] = *gasp* / OMG!