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Ironic isn't it? Kinda makes me think Gigaton Hammer should have had the added effect to be super effective on Steel types, but I guess the move is already busted enough as it is.
Tinkaton is definitely viable in OU but at the same times faces a lot of issues. It is on the weaker side and walled by a whole bunch of pokemon. Even worse, it gets set up on by things like Garchomp on sets with out Ice hammer and Dondozo.
Either Impish or Careful for me. First allows to counter Chien-Pao, second allows to check Iron Valiant really well. It obviously does more than that with Knock Off, Thunder Wave and maybe Rocks, but those 2 are the main appeals of this Mon in OU.