Pope Boniface VIII was looking for a new artist to work on the frescoes in St. Peter’s Basilica, so he sent a courtier out into the country to interview artists and collect samples of their work that he could judge. The courtier approached the painter Giotto and asked for a drawing to demonstrate his skill. Instead of a study of angels and saints, which the courtier expected, Giotto took a brush loaded with red paint and drew a perfect circle. The courtier was furious, thinking he had been made a fool of; nonetheless, he took the drawing back to Boniface. The Pope understood the significance of the red circle, and Giotto got the job.

— James McMullan

The story of Giotto’s O apparently dates from Vasari’s Lives of the Painters. First published in 1550, more than two centuries after Giotto’s death in 1337, Vasari’s profile adds this nice coda to the story:

This thing being told, there arose from it a proverb which is still used about men of coarse clay, “You are rounder than the O of Giotto,” which proverb is not only good because of the occasion from which it sprang, but also still more for its significance, which consists in its ambiguity, tondo, “round,” meaning in Tuscany not only a perfect circle, but also slowness and heaviness of mind.

Like being called “thick as a brick” today.