Sidestep his R (ram): he has to redirect it and that's the main knockup. If you dodge the return, he loses his engage.

Destroy his Q pillar before he E's into it — the pillar is breakable and gives him the knockup.

Ornn is weak in the early game against aggressive champions — punish him before he becomes an unkillable tank.

After level 14, respect his upgraded items and those of his allies — the stat advantage is real.