Ornn does not do juggernaut level damage. I was thinking that the lack of level 1 mr probably pushed him over the edge, but he most certainly does not have juggernaut-tier damage.
And "One of the best tanks in the game?" By what metric? If you mean cc potential, he has pretty decent cc but it's very conditional - not unlike Sion, actually.
If you mean that he has a powerful defensive steroid, yeah, his W provides very high windowed tankiness and the cc resistance is potential powerful, but he's pretty vulnerable when it's down -again, not unlike Sion.
His passive is ridiculous, but he also has some of the worst backline access of a Vanguard in the game, so he's pretty well limited in his potential for being broken.
Is he probably overtuned? Yes. Is he a genuine design issue? Probably not.