Once Upon a Time is a fairy tale drama TV series, which works in elements of various fairy tale & fantasy universes. So far, the series has featured several Oz cameos but no full inclusion in the story.
The little boy, Henry, has a storybook which has the fairy tale histories of all the residents of Storybrooke inside. On one page, an illustration of Winged Monkeys is seen.
In this Alice in Wonderland-themed episode, the Mad Hatter brings the Evil Queen with him to Wonderland. On the way, they pass through a room with doorways to many different worlds. One of the doors is green and bears the symbol of Oz.
Emma Swan, the show's protagonist, was given Ground Poppy Dust by Mulan to use against the Giant from Jack and the Beanstalk. It was able to immediately put him to sleep. Mulan said she had to go to another kingdom to get the poppies.
Quite the Common Fairy
Neal mentions Ruby Slippers as a portal to another world. Also, later in the episode, Regina suspects that Tinker Bell has put her to sleep using Poppies.
Regina mentions munkins in this episode
As of the beginning of the show's third season, Oz had not been visited and no characters from Oz made appearances, though Wonderland and Neverland both had been (the former now being the subject of a spin-off). However, the show's producers have stated in an interview that they do intend to visit Oz at some point, and that they have a way they intend to do so.
- Fables, a comic book franchise with a similar premise which also uses elements from the Oz series.