Given how toys is actually tegenau not tegennau like the course (incorrectly) says, is the plural for oranges orennau or orenau?
GPC says orenau; Gweiadur also has orenau; geiriadur.net says orennau; Bangor's dictionary doesn't have a noun oren (only oraens, for which it doesn't list a plural).
So take your pick as to which dictionary you believe....
(Personally, in case of doubt, I'd go with GPC.)
I can only remember seeing it as "Orennau" and that's how I would write it myself.
Thats how ive always been taught to say it too