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 18, 2007
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A greeting that has its origins in what is arguably the worst movie of all time: The Room (2003). This phrase is said about fifty times in the movie by various actors to various other actors, and very often at random, inappropriate times.
Johnny: I did not hit her, it's not true! It's bullshit! I did not hit her! I did NOT!! O hai, Mark.
by Leine-lu March 11, 2011
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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 16, 2010
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