It’s Mila Kunis.
Which is interesting given the description of her character from Oz The Great And Powerful press releases:
Theodora is a beautiful, naïve witch who is protected by her powerful sister Evanora. Theodora only wants peace to come to the Land of Oz and truly believes that a prophesied wizard will arrive someday to restore order.
Wikipedia goes so far to name not only Kunis as the Wicked Witch of the West but take the spoilers a step further by fingering Weisz as the evil witch Dorothy lands on with her giant ass… I mean… house…