urx unit loader Watch a sneak preview of Oprah on '30 Rock!'

Oprah‘s gonna be on "30 Rock" and she’s funny!

But she won’t do one thing for Tina Fey‘s character, Liz Lemon.

To find out what it is, watch the promo video.

Oh yeah, Jennifer Aniston‘s on, too. But the verdict is still out on her performance.