The confraternity's Oratory of San Giovanni Decollato, to the left of the main church facade, has Mannerist frescos by Francesco Salviati, Jacopino del Conte, both originally Florentine, and Pirro Ligorio, from the years around 1540.

The Archconfraternity originated in Florence and was named after the city's patron saint – its remit was to help those condemned to death, invite them to repent, give them the last rites and bury their bodies.

A small museum has various rather grisly exhibits from the execution process, including the basket that caught the head of the famous murderess Beatrice Cenci, beheaded in 1599.

The Confraternity "chose to make their oratory a show-piece of modern decoration", using Florentine Mannerist artists in a cycle on the life of Saint John,[3] beginning with a fresco Annunciation to Zachariah by Jacopino del Conte, perhaps in 1536.

Over the intervening years, "the Oratory became the most important collective artistic manisfestation of its time in Rome and the monument most representative of the emergence of Roman high Maniera style".

[10] Salviati was absent from Rome from 1543 and 1548, and on his return found that "gathering conservatism" and the "ever-rising concentration of authority in Michelangelo" put his "high Maniera" style out of favour.