I wouldn't say kass is OP. just annoying, if you send an AP champ midlane with him, he WILL get fed. if you see an enemy kass. improvise and rearange your roles. instead of sending a range AD bottom, send him mid, while your AP goes bottom. Kassadin can't do to much against an AD with his silence. and the rest of his kit is more of demolishing AP.
I've fought plenty of Kassadin's while playing, and I agree when I midlane with Brand against him, I hate it to death. But when i'm using a range AD. I suggest me going mid instead. Usually the AP will agree to this.