The Pokémon that appear in the wild can differ between Pokémon Omega Ruby and Pokémon Alpha Sapphire. You may refer to the version exclusive Pokemon by following the links.
Trading via PSS is a good way to fill your Pokedex and get new Pokemon. You can do this via different means:
Trade
Trade is a basic feature of PSS which allows players to exchange Pokemon locally (via infrared or wireless) or via the internet. Pokemon OR/AS players can still trade with their friends playing Pokemon X/Y directly once they (OR/AS players) have unlocked their PSS feature as part of the story after visiting Petalburg City.
Wonder Trade
This is another form of trading available in PSS which will allow you to instantly trade a pokemon of your choice to another pokemon offered by another trainer around the world. The catch is that you won’t know what pokemon you’ll get until the trade is already taking place. If you're lucky getting a generous trade partner, you may end up having a rare or powerful Pokemon early on.
Global Trade Station
GTS is like a trading auction where you can put up a pokemon you want to offer for trade and specify what pokemon you want in return. If a matching trainer is found anywhere, the trade will be completed. Unlike previous titles where you can only trade pokemon registered in your Pokedex via GTS, Pokemon X/Y will allow you to receive a new pokemon that’s not yet registered.