The book was illustrated by Baum's friend and collaborator W. W. Denslow, who also co-held the copyright. The design was lavish for the time, with illustrations on many pages, backgrounds in different colors, and several color plate illustrations. In September 1900, The Grand Rapids Herald wrote that Denslow's illustrations are "quite as much of the story as in the writing". The editorial opined that had it not been for Denslow's pictures, the readers would be unable to picture precisely the figures of Dorothy, Toto, and the other characters.
The distinctive look led to imitators at the time, most notably Eva Katherine Gibson's Zauberlinda, the Wise Witch, which mimicked both the typography and the illustration design of Oz. The typeface was the newly designed Monotype Old Style. Denslow's illustrations were so well known that merchants of many products obtained permission to use them to promote their wares. The forms of the Scarecrow, the Tin Woodman, the Cowardly Lion, the Wizard, and Dorothy were made into rubber and metal sculptures. Costume jewelry, mechanical toys, and soap were also designed using their figures.
A new edition of the book appeared in 1944, with illustrations by Evelyn Copelman. Although it was claimed that the new illustrations were based on Denslow's originals, they more closely resemble the characters as seen in the famous 1939 film version of Baum's book.