Oz is based on the location of the same name from the novel, The Wonderful Wizard of Oz.
A woodcutter and his wife are travelling to Emerald City when a cyclone drops a basket in front of them. The woman discovers an infant girl in the basket. Only the husband sees the newborn use powerful magic, and out of fear, he attempts to convince his wife to leave the girl behind, but she insists on keeping the child and naming her Zelena. ("It's Not Easy Being Green")
Years later, Zelena's mother passes away while the father grudgingly raises the girl. One day, Zelena is visited by Cora who seeks help from her to help cure her daughter. Zelena is then taken to the Enchanted Forest by Cora as Zelena agrees to help her. After helping Cora, Zelena and Cora's daughter Regina becomes friends. This results in Cora bringing Zelena back and forth from Oz to the Enchanted Forest for playdates. Zelena eventually finds out that she is Regina's older half-sister and Cora's first born, but in order to keep it as a secret, Cora removes her memories of such events and sends her back to Oz. ("Sisters")
Years later, Zelena's father admits her true origins. Upset, Zelena goes to the Wizard of Oz about her birth family. He reveals Zelena was abandoned because her mother, Cora, desired to become royalty, which her first-born child could not suffice. Zelena gets a glimpse of her half-sister, Regina, who became Queen and is being taught magic by Rumplestiltskin. The Wizard of Oz gifts her a pair of slippers to travel anywhere she desires, to which Zelena goes to Regina's palace. Later, Zelena, whose envy for Regina has turned her skin green, asks the Wizard to send her back in time to change the circumstances of Cora's abandonment. When told this is not possible, she angrily tears away the Wizard's curtain to reveal he is a showman named Walsh. Zelena makes use of the man by turning him into a flying monkey. ("It's Not Easy Being Green")
Shortly after, Glinda, the Witch of the South, arrives and asks Zelena to join her in Oz's sisterhood of witches. Along with the Witch of the East and the Witch of the North, they believe Zelena is destined to join them because of a prophecy foretelling of a sorceress arriving by cyclone and becoming the Witch of the West. Despite still obsessed about changing her old past, Zelena is convinced by Glinda of her destiny as a witch of the sisterhood. After receiving a pendant to harness her powers, they reach a girl, Dorothy, who arrives to Oz by cyclone. Fearing this child is the actual Witch of the West, Zelena's skin begins reverting to green and she discovers the prophecy also states the sorceress will "unseat the greatest evil the realm has ever seen", which she believes is herself. Zelena fakes her own death after purposely attacking Dorothy, who responds by throwing water onto her. Realizing the prophecy was right, Glinda offers Dorothy Zelena's place in the sisterhood, but the girl wishes to go home. With Zelena is gone, her magic is undone, thus the Wizard returns, or so Glinda believes. In truth, Zelena masquerades as the man to send Dorothy home. After the big reveal, Glinda vows to find another sorceress to fulfill the prophecy, but Zelena banishes her to the Enchanted Forest. ("Kansas")
Following Rumplestiltskin's orders, world traveler Jefferson heads to Oz in search of magic slippers capable of jumping through lands. Though unsuccessful, he does return with a crystal ball that Rumplestiltskin begrudgingly accepts. ("The Doctor")
With the Wizard under her control and Glinda trapped in the Enchanted Forest, Zelena takes dominion over the Emerald City and its guards. One day, a thief known as Robin of Locksley travels to Oz to acquire a potion called the Elixir of the Wounded Heart. He meets a young man named Will Scarlet, who helps him break into the Wicked Witch's palace. Zelena catches Robin, who narrowly escapes with some of the elixir. However, rather than taking it with him, Robin chooses to give the elixir to Will Scarlet. ("Heart of Gold")
Regina, using the apprentice's wand, brings a cyclone to take Zelena to Oz so she can protect Baby Hood. It breaks the glass of the clock tower. Zelena holds on to the clock and tries to not get taken away. She claims that it won't be the last they'll see of her, and she can't hang on and lets go. The cyclone then takes Zelena to Oz. ("Swan Song")