I find Nasus for example counters Ornn faily well in top, Mordekaiser looks like he does well as well possibly. The main thing you need is sustain because he has no burst damage. Almost all his damage is also Magic damage from his W amd brittled autos.... staying away from walls is also good, and if you can knock up, bind, or cc the Ornn before he can redirect his ulti for the knock.... he loses most of his ulti's use.
If they are played in support (like i find him awesome at) champions like Janna, Thresh, Soraka and Rakan can make his life hell as he tried to knock people around due to their ability to easily CC him and any poke he does is easily nullified or removed.
Ornn can not turn when he presses W so run behind him if you think you can kill him.
Titanic Hydra is good on him yes.