Yes, it is
You can actually catch/receive all non legendary Pokemon between ORAS and X/Y, making it possible to fill up the Pokedex, if you have all four of those games. (To my knowledge)
It will also require some breeding, evolving, and obtaining Pokemon from NPCs.
However, as I mentioned above, there are some legendary Pokemon that you can't catch in those games, these include Mew, Celebi, Jirachi, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, Genesect, Diancie, and Hoopa. (Note that these Pokemon can be obtained, but only by events, not capturing.)