Derived from the modern gaming term "hacks" or "hax". It is sandwiched in between two commonly know chatroom linguistics. "Omg" refers to the person becoming depressed or shocked in any manner. "Hax" refers to the situation or object being indescribably beyod ones imagination and "lol" is added to lighten the mood of the entire phrase.
There was a popular flash movie that showed how a message board thread becomes very popular when it's a girl posting to a bunch of male nerds. There was a picture of one such nerd replying to her post and saying "OMGHI2U!"
Outburst made by furiousmazda owners when their car mysteriously explodes
I started my car and a voice told me to get out. I closed the door and it took off by itself. I found it down the street securely placed between twotrees. I looked at the ground and exhaled. Later that day I told my mom what happened and she was all like "OMGHI2UWTFBBQ" the end.
In general it is a term used by adolescent males with raging hormones who see a picture of a physically attractive woman and think that that they will get somewhere by complementing them in such a fashion.
A "woman" posts a picture on, say, the TranceAddict.com forums, suddenly 40 young males quote the post with the pic and say "OMGHI2U!!!!!"