An Oz Key is a frequent item in Oz-related stories. Generally the key is gold or bronze, with the Oz symbol at the end of its handle. It is used for different purposes, but usually having to do with accessing an important Ozian treasure or getting into Oz itself.
In Return to Oz
In Dorothy and the Witches of Oz
The Wizard of Oz hid the Key to Oz with Dorothy, and she returned to Kansas, so that the Wicked Witch could not get it. The key opens a spell book which contains the Spell of Changing - the most powerful weapon in the world. The Wicked Witches try to get Dorothy to remember where the key is. (Dorothy and the Witches of Oz)
The Key was kept with the "Men of Letters" in their bunker. It was capable of turning any ordinary locked door into a portal to Oz. The Wicked Witch hoped to get the key to bring her armies from Oz into our world. The key was ultimately used to open a portal for Charlie and Dorothy to return to the Land of Oz with, after the defeat of the Wicked Witch. (Supernatural: "Slumber Party")