The United States of America, or just United States for short, is a country far away from Nonestica and hence Oz. Several Americans have made their way to Oz, however.
Dorothy Gale is from Kansas, and The Wizard of Oz is from Omaha, within the United States. (The Wonderful Wizard of Oz)
Billina also comes from America, and when she arrived there she started the first colony of chickens in Oz. (Ozma of Oz, The Road to Oz)
Dorothy also visited California, another state in the United States, on her way back from Australia. (Dorothy and the Wizard in Oz)
In Non-Canonical Work
People from Oz have also visited the United States. The Tin Woodman, the Wogglebug, the Scarecrow, Jack Pumpkinhead, the Sawhorse, and the Gump visited the U.S. to pay Dorothy a visit for her birthday. They first arrived in New York State, where they met Jimmy Jones, and then they began traveling west towards Kansas. (Queer Visitors from the Marvelous Land of Oz)