Onion John is a humorous and fastmoving story peopled with vivid and lively characters. Written from the point of view of twelve-year-old Andy, the novel uses realistically boyish language; casual humor and colorful descriptions engage the reader in the actions and emotions of the story. The story includes bits of trivia, such as information about the origins of Halloween, which add to the lighthearted mood. The balanced plot unifies the novel; questions raised in the opening chapters are repeated and answered in the concluding pages.

Krumgold uses irony, particularly in relationship to Andy's father and Onion John, to reveal his characters' different attitudes. Mr. Rusch believes that the greatest gift he can give his son is to enable him to "go to the moon," while Onion John interprets that wish as an immeasurable insult: "I've never heard of any father who would send his only son...