From Professor Sycamore after completing the Kalos Pokédex (excluding Articuno, Zapdos, Moltres and Mewtwo)
A Youtube Video showing this
Also in the youtube video's description;
Head to Coumarine City, talk to the Game Director then, when you're done there, go to the Pokemon Lab in Lumiose City. On Floor 3 of the Lab, speak to Professor Sycamore and if all 3 of your Pokedex are fully completed (caught all Pokemon), he will give you the Oval Charm.