The actor who played the Powerful Oz in the movie played various other characters in the film, including the role for Professor Marvel, who lives in Kansas and appears in the beginning scenes of the film.
The costumes for the characters changed a lot, including those of Professor Marvel. At some point, they decided that they needed an old and shabby looking jacket for the professor.
The producers went to a second-hand clothing store on Main Street and bought an entire rack of coats. The wardrobe manager and the director decided on one they liked. When they looked inside the coat, they noticed that it said "L. Frank Baum" the name of the author of the Wizard of Oz book!