I seriously doubt he is the Wizard of Oz…who is an old man from Kansas and not a real wizard at all. He isn’t even too terribly bright. Plus, I don’t think the writers would have August be a completely random out of the blue character from a realm that the show hasn’t even touched on beyond that green door we saw in Hat Trick. If we see Oz at all, I wouldn’t expect it to happen until next season- within it’s own new storyline.
August has to be someone who was close to the royal couple when the curse was enacted…very few characters knew about Emma.