Conversation so far:
What was Judy Garland's role in The Wizard of Oz?
Judy Garland played the main protagonist, Dorothy.
Did Judy Garland sing any songs?
Judy Garland sang the song with which she would afterward be constantly identified, Over the Rainbow.

Question: What was The Wizard of Oz based on ?
Self-contained rewrite: What was The Wizard of Oz based on?