Omigod

Is it pronounced the same way as 'Oh my God?'

Yes, but it's spelt OMG. I've never seen it written like that.

Thanks. I've seen it written like that in many different places. Including:
http://www.urbandictionary.com/define.php?term=omigod

http://dictionary.reference.com/browse/omigod

Actually, "omigod" IS pronounced with a short I and replicates the reduction that happens when you condense it all into one word (it's a sort of slangy term). OMG stands for oh my god, which would give the full long I sound to "my". They aren't the same thing.