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 furious mazda 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 two trees. 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!!!!!"
It is like when you join some dumn IRC channel. It is all like OH MY GOD! HI TO YOU. Converted into some shit like omghi2u has made people in spelling asses. Results of this omghi2u has created a new level of internet assmasters. Like those of #omghi2u on progameplayer IRC network.
*** JACKASS has joined the chat
<SOMEONE> omghi2u
<JACKASS> omghi2u2
<SOMEONEELSE> yeah, omghu2u2, we're 1337