The
Wizard's full original name was (deep breath)
Oscar Zoroaster Phadrig Issac Norman Henkle Emmanuel Ambroise Diggs. When he grew up, he shortened this unwieldy name to
Oz - after all, would you want your initials to be
O.Z.P.I.N.H.E.A.D.? While working as a stage
magician,
ventriloquist, and
balloonist for a circus, an accident brought him to
Oz. Since the balloon had his abbreviated name - also the name of the land - on it, all the residents assumed he was the new ruler.
As revealed in
Dorothy and the Wizard in Oz.