Oz is an allusion to L. Frank Baum's fantasy land in his popular book series which begins with The Wonderful Wizard of Oz. An allusion is a reference to something outside the story that is assumed to be well-known by the reader. In The Wonderful Wizard of Oz, the main character Dorothy learns more about herself after her adventures in the fantastical land. While there, she and her compatriots (the Scarecrow, the Tin Man, and the Lion) are told by the Wizard of Oz that the things they seek were inside them the whole time.
Much like Dorothy, Summer is an orphan living with her Aunt and Uncle. And just like Dorothy traveled to Oz, Summer and Uncle Ob travel to Charleston, West Virginia in Missing May. Their adventures there help the characters grow and realize their true potential. This is how Uncle Ob finally realizes that he should act as a guardian and parent for Summer. It is here that Summer regains her ability to act like a kid and reclaims her childhood. Uncle Ob's ability to be a parent, and Summer's ability to be a joyful child, were traits that were in them all along; they just needed a trip to reveal those abilities to themselves. In this way, Rylant draws similarities between Charleston and Oz.