From my mememory, someone said something then someone replid to it with just an oh.

oh

As far as I can remember, it was Naru that started it by simply replying "oh" to a post, and me and a few others (I think the shitposting triad) found it funny and perpetrated it by spamming it and blank quotes everywhere. I guess the excessive usage made it take off into a psuedo forced meme of sorts.

Correct me if I'm wrong, but I'm sure that's what happened.