Orcus is one of the old gods of Latium and predates the Greek influences, just like Pomona and Tellus. The very fact that a second god, Pluto, is associated with many of the same things, is probably because Orcus predates the Greek influences.

Also, the Greek word begins with aspiration, which would mean that it would likely have been Horcus in that case (case in point - ωρα becomes hora). I'm not going to speculate on whether or not they might both derive from the same Indo-European root, but it seems safe to say that the Latin term did not derive from the Greek.