I feel like the ornn nerfs on the PBE are over tuned for what they are trying to nerf. Ornn is already in a pretty balanced spot right now, i know it may not seem like it but, the numbers speak for themselves
At the time of posting, 2 of these links are at ~48% win rate on ornn.
What makes ornn unique and viable as a tank is his unstoppable. It ties in nicely with the rest of his kit and makes him a good tank. His ult nerf is justified and is a perfectly acceptable nerf.
Even if you disagree i feel that with the upcoming nerfs to the resolve tree ornn will then be put in a balanced spot because at least 1 of his top tier runes will be put in place
Armor and MR changed from [20 + 30% ] to [85 - 175]
Explosion damage lowered from [40 - 140 (+3.5% of your max health)] to
[10 - 120 (3% of your max health)(+15% of your bonus attack damage)(+10% of your ability power)]
I do however think that he needs a nerf. his W is a little over powered but its not because he becomes unstoppable, i feel like its because its oppressive in lane. 6 Seconds with a buff that will mess you up if you go in for CS.
Let the Brittle buff timer scale with levels. I feel like this will remove Ornn's oppressive early game nature.
Brittle early damage down but leave late game scaling as it.
If you are insistent on removing the unstoppable from his W, maybe you should put it on his E or his R-recast. Maybe both because this specific W nerf is relatively strong
Hope this helps and puts things into perspective. See you on the fields of justice.