Any Rayquaza that knows Dragon Ascent can. My V-Create Event one can Mega Evolve. An Old Woman at Meteor Falls can teach Dragon Ascent to any Rayquaza.

Are you importing your Rayquaza from X or Y into ORAS?

You can have your shiny Rayquaza "relearn" Dragon's Accent to mega evolve. Just go to the move reminder and make sure it's at least above the level of the in-game Rayquaza.

Edit: or what rocky said

I passed over all my shinies from X. Using a shiny Mega rayquaza was one of the things i was hoping to do most when it was announced.

Oh, that's nice. I didn't wanna import any Pokemon from my X copy since I like to start fresh and stock up on Pokemon from that respective game.

But, there was that one time when I imported my Blaziken from my Black 2 into my Pokemon X game. That was the only time I did something like that.

Almost done transferring my competitive Pokémon to Omega Ruby. Just one more row to go.

Also this:
http://i2.kym-cdn.com/photos/images/...69/286/73d.gif
This always cracked me up. :lol

That's an Ace attorney breakdown material right they're. :lol Somebody needs to make that.

That Archie GIF though! :lol

How does Master Archibald flip out?

Meant to say Maxie, my mistake. I always confuse the two names regularly. Their names are really similar.