Wicked: The Life and Times of the Wicked Witch of the West
Oatsie is the guide the Maunts hire to take Elphaba out to the Vinkus region. She seems very spunky and funny, and it's through her that we get some insight into how Elphaba has changed since we've last seen her. We also learn about the new character of Liir through her.
Oatsie has a pivotal scene where she details some legends about the Kumbric Witch and speaks at length about themes of good and evil. Oatsie's spiel here applies directly to Elphaba's future as a Witch, as well as to the novel itself.