I am maining midlane and have trouble with Orianna.
I've played different champions against her with mixed success.
Karthus is quite good against Orianna if I dodge her poke and just farm from range with q, but well yes, he's always a safe pick.
I've tried different AD Mages including Jayce, Trynd, Panth, and Talon against her and all seemed to work out pretty well but mainly because Orianna went MR.
How else can you counter Orianna?