well, it literally means "hi" in "lolspeak". what's lolspeak? familiar with the internet sensation of lolcats? if you aren't, i'm not sure you are missing out truly, but i personally think you are. basically, just images of cats with silly titles.
not a cat person? there are loldogs too. i love those. especially when they involve corgis or bassets
so those are the origins of "oh hai." i also enjoy it because it does sound like the state i grew up in (ohio) and the japanese word for good morning (ohayou - pronounced similarly to ohio) and is one of my favorite phrases from japanese class. that's just random little tid bits, but mostly, it's all about those lolcats.
strangely, i'm not a cat person.