Probably their own category...there are quite a few.
Of course, this raises the ever-annoying OVA vs. OAV question...a trifling detail if there ever was one, but one has to be picked for the sake of organization. According to this OAV is the original acronym, whereas there was a shift toward "OVA" in Japan in order to avoid confusion with adult animation. Technically both are right. Some say OAV is more common among us English speakers...though this seems subjective. Maybe the apparent industry standard should prevail?