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BTW, full compatibility means that Pokemon/items can be traded between the games without the need for pokebank
Yes and No
It is made clear that you can transfer XY Pokemon to ORAS.
However it is not clear how exactly.
Jinichi Masauda has confirmed that no patches will be made so that ORAS and XY will be fully compatible like Soulsilver and Platinum.
So if you trade Diancie over to ORAS you cannot get it back.