What I think made the "unstoppable" feel so good is that it feel really thematically satisfying. Ornn, like a lot of tanks, feels more appropriately themed as a Juggernauts - rather than being a powerful being that decided to focus their power on defense and protection (such as Leona, Taric, Galio, Etc), he feels like a massive, terrifying, slow, inexorable force of nature.
It's not that he's too big to easily kill, but you can survive his attacks, it's that he's slow and unstoppable, and if you aren't careful he might catch up. So be careful, and keep some distance.
Playing as, he felt like a slow, clunky, bludgeon, that you used to club overaggressive enemies into submission (which by the way has always been something I think should be valued more, I used to love playing old Shen, Alistar, and Taric top to stalemate the lane. I know "passive lanes" are considered bad gameplay, but I miss having champion picks you could choose to punish aggression, when you know the enemy Renekton or Riven won't be able to resist diving head-first into you). Playing against him, as long as I didn't pick a champion reliant on lane-bullying, I felt that I could keep some distance, heal up his relatively minor poke, pick smart trades, and scale up. As long as I accepted that he was a stronger early laner than me, I was fine. I think people really struggled against Ornn because he was a tanky lane-bully, and they're not used to playing back against tanks.
So, to actually get to your question, I think it would feel good to double down on that fantasy of "Approach cautiously, or pay the penalty". Keep him slow, clunky, and with low kill pressure, but give skilled players the tools to punish blind aggression hard.