Aggron in OU? Yeah I know, it's trite saying that he cannot be OU, but think about it. Remember when Relicanth got Head Smash in Platinum and there were so much hype about it? Now he's listed as NU in Smogon which goes to show that you can have 225 base power moves but still fail if you have bad stats and typing.
Relicanth and Aggron don't differ much in stats and movepool BTW. Aggron has 20 BP more attack, but that doesn't matter much considering that it's that 225 move power that's factored in more into the calculations, and the 20 more BP Aggron has only make a slight difference. Plus, Relicanth has a better typing (4x weak to grass as opposed to 4x weak to fighting and ground), better HP (with the same SpD, thus taking special hits better), and 5 point better speed (which actually makes a huge difference). And Relicanth also knows EQ, Aqua Tail (but STABed, thus better), and Rock Polish.
So yeah, I don't see how this situation of "Aggron is OU" any different than in Platinum when Relicanth was hyped because of getting Head Smash.
a few differences make a huge impact
water/rock is a terrible defensive typing, it is weak to:
grass, electric, fighting, ground
and resists:
Normal, Poison, Ice and Fire
Steel/Rock is weak to:
Fighting, Ground, Water
and resists:
Bug, Dark, Dragon, Flying, Ghost, Ice, Normal, Poison (Immune), Psychic and Rock
4 weakness and 4 resists vs 3 weaknesses and 10 resistances is a big difference, sure, two of the weaknesses are 4x, but any EQ that kills gron kills Relicanth, same with Close Combats, Aggron has the option to turn one of his weaknesses into an immunity though, which relicanth can't do. and while anything might pack HP Grass or Electric (for pert or coverage), you can be pretty sure of what's packing EQ, Fighting and Water
Aggron might have 30 less base HP, but it has 50 more base Defense, which more than off sets it, and while an extra 20 may not seem like much for the attack stat, it is enough to ensure 2HKOs on damn near anything, and allows for a few important OHKOs that Relicanth doesn't get.
Relicanth has the bonus of STAB Aqua Tail and slightly more speed, but Aggron has some more useful perks in his attack, defense, typing (which isn't as bad as most people make out, Dragon resist is extremely useful), SR resistance and Toxic Spikes immunity (a godsend vs stall) and Magnet Rise