It's Ornn's hitboxes that are broken, not his kit

Ornn's combo is impossible to counter or outplay once he starts it. The sheer size of his E and W hitbox means that he can knock you up and apply Brittle so long as you happen to be nearby a piece of terrain. His cheap mana costs and the ease with which he can acquire a Glacial Shroud means that he's a brick wall in lane and can repeatedly bully you out with his foolproof trading combo. I play Ornn. I've applied Brittle that were to the side of me, behind me, and even to people that aren't being touched by the visual indicator of the fire, which is total bullshit. Ornn's Brittle mark is absurdly strong for trading because it deals a large amount of damage, and gives free disengage in the process when he knocks you up/back. Opponents should be rewarded for correctly sidestepping the Brittle mark in order to avoid the damage it provides.

Ornn needs further nerfs. This is clear. If Riot is unwilling to hit the ease and potency of his trades then they need to shrink his hitboxes significantly because they are way out of line. His E and W hitboxes are too generous for the power they provide, and combined with an already massive hitbox on his ultimate means that Ornn is extremely foolproof and has little to no true losing matchups (the only real losing matchup I can think of for Ornn is Darius and a couple ranged characters, which he can still turn around later in the game with better macro play; the rest he wins or goes even), an unhealthy trait to have in such a polarizing top lane meta.

Another thing that might need to happen is his W damage being shifted to a flat value that scales with AP, and more power being placed in the Brittle mark (along with a hitbox nerf to W so that opponents can sidestep the mark to outplay Ornn's damage). Ornn's damage being relevant no matter his success is bad design....even Tahm Kench, the only comparable champion with a large chunk of his opponents max health, has his Devour on a much larger cooldown, high mana cost, and requires three stacks to use. He is also heavily slowed, not unstoppable, and is giving up allied utility for swallowing an enemy. Tahm's damage is also dependent on his ability to accrue health items and secrure a lead in lane - if you put him behind, he will be doing noticeablely less damage due to a gold deficit. While Bellow's Breath and Devour are two very different abilities, the comparisons between them and the two champions who possess them show why the former is unhealthy.