To me it just looks like a buildup of old skin. When they grip the perches or walk on a flat surface that is the area that doesn't make contact because of the nail, and so never wears down as the rest of the foot does.
It doesn't look like mites, infection or bumblefoot, and it doesn't seem to be causing any problem to Oreo so just keep an eye on it.
Gosh he's handsome!