Based on The Wonderful Wizard of Oz (1900), The Marvelous Land of Oz, Ozma of Oz, and The Emerald City of Oz (1900-1920) by L. Frank Baum (1856-1919). (Source: AniDB)
Watch The Wonderful Wizard of Oz episode 1 online — subbed and dubbed, with automatic AniList / MyAnimeList progress sync.