A rather pointless bastardization of the existing word "oh" (in the positive sense).
It practically has the same meaning as "ohh!", "ooo!", and "ooh!" (usually with more o'
s and/or h's attached), just that it's meant to
sound more enthusiastic and pleasing, I guess. In the same fashion, "ou" is usually followed by more u's at the end, depending on how excited you want to sound.
Notes about usage: ou and
omg (usually with many g's) convey about the same degree of zeal, and therefore can be swapped. The only difference is that
omg sounds more fitting while ou sounds more
cringe.