Ok first off I used to play orianna and zed A LOT. It is pretty easy to counter a zed with them. Cause guess what? Zed shows up right behind you when he ults. You know exactly where he is gonna be right when he lands. Now it comes down to your reaction time vs. the Zed's.
Also, with Brand, you can stun him. With Karthus well you're shit out of luck cause he is immobile without any kind of stun or snare. But that's the price you pay with Karthus. Viktor can just put a w down as soon as zed starts to engage, then you can blow him the fuck up.
Varus has his ult to root zed. He is a poke based champ, so as long as you can land your q's, you should be fine because Zed will have such low health that if you hit a combo on him, you win.
Sounds to me like you just happen to not want to use counterplay on zed and would rather act like he's not there, in which case it is 100% your fault. I've seen countless Oriannas destroy zeds in lane. Same with Luxs, Brands and Viktors. If you happen to pick Karthus or Varus against him, well thats too bad.
But a small handful of champs that one champ counters isn't op. Its called balance. Get over yourself and learn to counter. Time to take your training wheels off.