Deposit the Pokemon from XY to Pokemon Bank. Then save and disconnect from Pokemon Bank and put ORAS in the Game Slot. Go back to Pokemon Bank and take out the Pokemon from Bank.
You need 2 3DSs for this
It's like how you Poke-Transfer from Pokemon HGSS/DPPT to BW/2. You can trade from XY to ORAS via Infrared.
Without Pokemon Bank and 2 3DSs look here.
It's a little risky in my opinion but its worth a go.