In an interview that the Dispatch shared with Eddy Kitsis and Adam Horowitz episode 16 titled "It's not easy being green" will bring light to Zelena's past when Glinda the Good Witch shows up. The two say that fans will understand when they see her back story and learn more of what happened to Zelena in Oz and as a child.
Regina and Zelena will definitely get into it. Zelena's problem will be a global and internal problem, and not even have much to do with Regina specifically, but the vengeance she wants is exactly what will be the focus of the coming episodes of the remaining season three.
While Oz itself will be important, it isn't going to be a focal point where everyone will pick up and go to Oz. What happened there and the significant characters that we meet from there are going to be more important than the land itself. Zelena will be a huge driving force in the second half of the season and she is going to be the biggest antagonist that will make the Evil Queen look nice.
Are you interested in learning the Wicked Witch's story and why she is so mean?