Osanna was the daughter of the nobles Niccolò Andreasi, whose family had originated in Hungary, and of Agnese Gonzaga. She was reported to have had a vision of angels at age six. Feeling called to consecrated life, she rejected a marriage arranged by her father. Unable to explain her attraction to religious life to her father, in 1463, at the age of 14, she secretly received the religious habit of the Third Order of St. Dominic. She had been drawn to this Order from her admiration of two members of the Order, Catherine of Siena, and her contemporary, Friar Girolamo Savonarola, who both represented to her lives of strict self-denial.