Keys of Oz
The six keys of Oz were forged from Oz steel. They can only be repaired in that magical realm.
The Keys of Oz are special magical keys, forged of Oz steel. If one is broken, it can only be repaired in Oz itself. There are known to be at least six keys of Oz, with only the properties of two having been revealed thus far. The six keys remain in the Men of Letters Bunker.
Key to Oz
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. It was first discovered by Men of Letters member Clive Dillon, who used the key to go to Oz, leaving it behind on Earth. Author and member L. Frank Baum would go on to find the key and go on a rescue mission to retrieve Dillon, which led his daughter Dorothy Baum to stow away and get trapped in Oz for a time. The key was stored away in the Men of Letters Bunker by Baum for the good of Oz.
Eventually, the key was damaged by Dark Charlie to prevent Good Charlie from going back to Oz, and Clive Dillon confirmed that it can only be repaired in Oz. The remaining fragment of the key remains in the Bunker.
The only other method of entry into Oz is through natural disasters such as tornadoes, the eye of a hurricane, or whirlpools. The Wizard of Oz was shown to be powerful enough to cross over from Oz through a mirror.
Dean found the key while he was looking through the various artifacts in the Bunker and kept it in his room.
After seeing a picture of the key in Dorothy's journal, Dean recognizes it and goes through a chest in his room until he finds the key. However, the Wicked Witch of the West manages to take the key and kill Charlie. The witch intends to use the key to bring her armies to the Winchesters' world, but her plans were foiled by Charlie who was revived by Gadreel. Later, Charlie and Dorothy travel to Oz to end the war.
The key was broken in half by the dark version of Charlie Bradbury to prevent Good Charlie from going back to Oz. The Winchesters try to find a way to repair the key to get to the Wizard of Oz, but Clive Dillon confirms that the key can only be repaired in Oz. He summons the Wizard to them by mortally wounding himself, causing his dark half to come to save them both.
Inner Key of Oz
The Inner Key of Oz is a mystical key that opens a door to a person's soul and lets the darkness out. This causes the person to physically split into two separate beings -- a "good" version and a "bad" version. The two entities, however, share a connection. Any harm inflicted on one is felt by the other, ensuring that the dark entity cannot kill the good.
Upon Clive Dillon's venture into the realm of Oz, he was grabbed by a coven of witches who used the key on him, unleashing his id. This dark version of Dillon was a power-hungry man who killed all the witches and in his madness declared himself the Wizard of Oz, ruling over the realm in the shadows.
When Charlie Bradbury left for Oz alongside Dorothy Baum, she was met by war. During the war for Emerald City, the witches were about to take the city until Charlie made a deal with the Wizard of Oz: he offered to use the Inner Key of Oz on her, telling her for them to win she would need to unleash her inner darkness. The tide of battle turned with Dark Charlie, who would go on to single-handedly win the war for Oz. The awful tactics that Dark Charlie employed to win the war caused a rift between the two entities, with Good Charlie not wanting anything to do with Dark Charlie. The two Charlies are eventually made whole when Charlie uses the Inner Key of Oz once more. The Inner Key of Oz along with the other five keys remain in the Bunker.