Constantly adapt your positioning in lane so Ornn is unable to knock you up with his E. Stand away from walls and his Pillar Q so he is unable to CC.
Refrain from using any major CC abilities on Ornn when his W is off cooldown as he can use it to dodge or escape CC.
Once Ornn hits level 6, his kill pressure in lane will increase and he may start getting ganks from his Jungler. Get vision in the river so you can spot the Jungler early.
In team fights, avoid grouping too closely as Ornn can use his Ultimate R to get a multi-person knock up and start the team fight.
Ornn has the ability to knock up enemies that are close to his Q, terrain and walls. Avoid fighting inside the jungle as it will allow him to land an easy knockup.
Ornn will tend to stick in a side lane when no team fights are occurring and when there are no major objectives up. Use the numbers advantage to start a team fight while he is away from his team.
Once Ornn unlocks his Ultimate R, he will have an easier time in lane. He will be able to look for kills and even set his Jungler up once he has it available to him. Expect him to play more aggressive post 6.
Every time Ornn gets an item upgrade, he will be much stronger as each unique upgrade offers him extra stats.
Ornn is good in team fights. Try to fight when he’s not there as it will make team fighting much easier. Also, setting him behind in the early game will make him less strong in team fights.