It is an acronymn of broken English.
It says in long form:
"Oh My God! Hi to you!"
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!!!!!"
Oh my God, hi to you!
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!"
o my god hi to you...
used by cool guys when they meet a chick on the net
cool guy: 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
<SOMEONEELSE> yeah, omghu2u2, we're 1337
Used to make fun of the person it's being said to, a mimicry of 11-year-old girls talking on AIM.
Tard: "i was like walking to the store and this really hot chick was like 'omg, i r like u', and i was like sex0rd her"