The Wizard of Oz is a classic fantasy experience that follows a young girl's journey through a land that is as beautiful as it is dangerous as she tries to find her way back to Kansas. On her adventure, she also meets the dangerous Wicked Witch of the West, who wants her deceased family member's ruby slippers and to kill Dorothy for taking them. But the question of who the Wicked Witch of the East is is never answered, and one theory believes that she was actually the film's main character all along.
When Dorothy awakens from her dream, she comments on how three farmhands who work with her Aunt and Uncle were actually the Lion, Scarecrow and Tin Woodsman. While the film ends with Dorothy believing she just had an imaginative dream, Reddit theorist, Primetime22, believes that her dream actually happened. But rather than it is a make-believe world, Dorothy actually traveled to another reality where she represents the Wicked Witch of the East.
The Oz book series further explores Dorothy's adventure in the land of Oz along with other characters from the franchise. Since Oz seems like a real place in the series, it's possible that the cyclone that takes her away from Kansas was actually just a portal to another universe. With Oz's counterparts being people she knows in real-life, the theory begins to get more legs. That said, the most significant piece of evidence to support this comes from the Wicked Witch of the East.
When Dorothy arrives in Oz, her home falls on the witch, killing her instantly. This frees the Munchkins from the wicked witch's tyranny, and not long after, Glinda the good witch arrives. With her magic, she transfers the witch's ruby slippers to Dorothy, instructing her never to take them off as they are very powerful. However, the underlying reason for this is likely because Dorothy is the Wicked Witch of the East in this reality, and, should the two exist in the same world, it could create a paradox. As a result, her death opens Dorothy up to take her place in the world, and the slippers are more easily transferred to her feet as it's like they never lost their owner.
This also adds more context to the witches of Oz, as it shows the Wicked Witch of the East as someone who always wanted her sister's slippers, and just as she can get them, another version inexplicably appears and takes them instead. It paints her as more bitter and petty rather than angry and mourning, which works much better for the witch. The shift also shows how Glinda's main goal is to maintain balance as she knows the slipper's powers and knows the best place they can be is with the person who wore them in the first place.
The Wizard of Oz never adequately explored the lore and rules of Oz. As a result, any reality or theory has a glimmer of possibility. Regarding Oz being a parallel reality, it makes an incredible amount of sense. With Dorothy being a version of the Wicked Witch of the East, it explains why she's so essential to the story and why her quick thinking could kill her multiversal sister once and for all.