Diana Issues?

This is not an OP rant. Rather, this is where I want to learn how to beat Diana. In my most recent game, I played as Orianna against Diana, and everything pre-6 was fine and dandy. Once Diana hit six, however, the nightmare begun, as she killed me under tower while I has half life, then started roaming bot to lynch their souls. From what I had gathered, Diana was able to one-combo kill me at level 7 after I had bought Chalice. Even with extra magic resist (going up to 90), me and my team were still being deleted. There was almost no reaction time. She didn't even land her Q half of the time, just a hop, skip, and a sword to the gut. So what may I do next time? Should I call my jungler to camp lane? Should I pile on MR? Should I disconnect when I see she is my opponent? Or is she actually OP? I want to know.
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