Oscar Diggs, a circus magician from Omaha, Nebraska, traveled in a hot air balloon marked "OZ" (his first two initials) before a storm swept him away to the Land of Oz. Arriving there, he proclaimed himself a powerful sorcerer and seized rule of the Emerald City, sustaining this illusion through elaborate tricks and props.

To maintain his formidable image, he adopted different terrifying forms for Dorothy and her companions: a giant head for Dorothy, a beautiful lady in green silk for the Scarecrow, a monstrous beast for the Tin Woodman, and a ball of fire for the Cowardly Lion. When they demanded he fulfill his promises, he tasked them with defeating the Wicked Witch of the West. After their victory, Toto tipped over a screen during an audience, exposing Diggs as an ordinary man. He confessed to being a fraud with no real magic.

Attempting to return Dorothy to Kansas in his hot air balloon, he departed prematurely when Toto chased a mouse, leaving her behind. This failure prompted his exit from Oz, and he entrusted the Scarecrow with ruling the Emerald City.

Back in the United States, Diggs resumed circus work. Dorothy encountered him during a circus visit in Kansas; he recognized her but warned against discussing Oz, as others dismissed his stories. He expressed nostalgia for Oz yet accepted his return to ordinary life.

Later, after Princess Ozma was restored as Oz's rightful ruler, Diggs returned permanently. Under Ozma's decree, he became an apprentice to Glinda, the Good Witch of the South, to study magic legitimately. Ozma's laws restricted magic use primarily to himself, Glinda, and herself. He shifted from illusions to inventing useful mechanical devices to aid Dorothy and her allies during their adventures.

In his later role as Ozma's advisor and court magician, he focused on non-magical ingenuity rather than sorcery. This transition marked his development from a deceptive ruler to a valued contributor to Oz's society, applying his skills for practical benefit.