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!!!!!"
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