The 31-year-old actress, who plays Glinda the Good Witch in the Disney movie directed by Sam Raimi, donned the revealing Prada gown which was split to the waist on one side, exposing the star's black underwear as she graced the yellow brick carpet.
Williams, currently dating actor Jason Segel, teamed the Swarovski-crystal detailed dress with matching Prada heels and a Kate Young for Target clutch.
The starlet was joined by equally as glamorous co-stars Mila Kunis, Rachel Weisz, James Franco and Zach Braff.
Take a look at the Oz The Great And Powerful premiere here:
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