As ornn, Proper turret placement and spacing is vital towards winning this matchup. Make sure that Ornn can't pin you against terrain either.

Keep poking him regularly in this matchup to hinder his all-in potential as much as you can.

If he has used his ult, and he is about to send the ram towards you, CC him immediately. His ult will go to waste this way.

Keep the wave close to your tower. The enemy has no sort of sieging potential and can be easily exposed to ganks. As ornn, The proper location and spacing of the turret are essential to win this matchup. Make sure that Ornn can't put you against the ground either. Keep trapping him regularly in this matchup to hinder his full potential as much as you can. If he used his ult, and he's about to send the ram to you, CC him immediately. His ult will go to waste in this way. Keep the wave near your turn. The enemy has no kind of sifting potential and can easily be exposed to the ganks.