A fully capital OWO, unlike a lowercase owo or mixture of the two (OwO) is an English word that is an initialism of "Oral Without". In prostitution, this signifies performing oral sex on a man without using a condom.
This isn't the reason the weebs and the furries started using it, but it sure is a strange coincidence that the most
popular use of it is the "OwO what's dis?"
meme, depicting a
furry finding a bulge in someone's pants.