The phrase is an intentionally misspelled "Oh hi" and is usually used as a title on cutesy pictures of animals like on lolcats.

The phrase is often followed by a short, badly spelled sentence as if the animal/subject in the picture is talking to the viewer.

O Hai may also refer to these types of cutsey animal images in general.
Picture of kitten in a stack of business reports.
"O Hai, i am bein yur nu secritary. Get me cofy."
by idunnoit August 19, 2007
Another way of saying "Oh, Hey there!"

As started on the freenode network channel #tyreus
Ambur has joined #tyreus
<Brad_R> O hai thar, Ambur!
<Ambur> O hai!
by BwaddArr / Monkie / MrMonkie January 17, 2010