Cho is OP, here's why:

1. His passive allows him to never have to leave lane. With the immense health and mana regen, he's an immovable force.
2. His Q (which isn't that hard to hit) has a 1:1 AP ratio. That's a large AoE knock-up with a 1:1 AP Ratio. That's Veigar's W, except it CC's your opponents.
3. A 1.5 to 2.5 second cone silence that stretches past minion waves and straight into your enemy, dealing hefty damage and preventing them from being able to fight back, and removing any character that uses abilities from close-quarters teamfights.
4. His E gives him up to 80 + 30% AP bonus damage to his auto-attacks that pierce his targets. By building AP, Cho'Gath is making his auto-attacks impossibly powerful as well
5. 1k True damage nuke to his opponents with only a few AP items, because why not?

Also, Rylai's allows his E to slow his opponents as well. Everyone who's hit by it.

What I think should be done to fix it:

Nerf ChOP'Gath's Q AP damage by 30% and W silence by about a second to a second and a half. In fact, it wouldn't be a horrible idea to just make it a slow, honestly. You can't fight back against the Freelo of The Void, especially if he's building AP. There's no counterplay there. Heck, he can W flash and R enemy AP/ADCs and kill them while they try to run away (but they can't flash or cast spells because of that insane silence + silence damage. If he lands that with Rylai's, he'll get a 100% guaranteed Q as well, just burying the nail further into the coffin.

If he lands Q alone, that player is dead, especially in the mid to late game and don't even get me started on how much more powerful Cho becomes when he has a team mate who can engage first and have Cho on follow-up duty or about how amazing he is at being a peeler for his team.