A pointless bastardization of the existing interjectory 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 sounds 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.
Most exclusively used by Gen Z age groups, especially on platforms like
discord.
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.