OMGWIN can be understood by 'Oh My God, that is so awesome'. Where 'WIN' means that it beats all other things when compared to it - amazingly awesome.
WIN! WIN! WIN!
Also used sarcastically when things are completely lame - absolutely, and obviously not awesome.
OMGWIN! I just sold a PlayStation 3 for $2000!
Dude, when you were out of town the other day, and I swung by, your sister invited me in to her girlsy only pool party. It was pretty cool and everything, 'till we and all her friends started topless chicken! It was SO OMGWIN!
When will the omgwin! end?
(realizing you just locked your keys in the car, while it's raining, and running, on the side of the freeway) .......... omgwin.