Keys of Oz
From Super-wiki
The Key to Oz is a mystical key, that when used on any door, turns that doorway into a portal to the Land of Oz. The key was stored away in the Men of Letters Bunker by author and member L. Frank Baum, for the good of Oz.
The only other method of entry into Oz is through natural disasters, like tornadoes, the eye of a hurricane or whirlpools.