Can you battle someone with ORAS when you have XY ?
You can with most Pokémon, however there are a few execptions.
Groudon with Precipice Blades
Kyogre with Origin Pulse
Rayquaza with Dragon Ascent
Pokemon holding items available only in Omega Ruby and Alpha Sapphire, like the items for the new Mega Evolutions.
Of course Smeargle applies to this as well.