The paper aims to demonstrate that canon of Milan’s cathedral Ottaviano Abbiate de’ Forieri (1546-1615) should be considered as a close and important collaborator of Carlo Borromeo. To do so, this work focuses on Ottaviano’s family, academic education and pastoral activities during Borromeo’s episcopacy (1565-1584) investigating unpublished or little-known archival documents housed in Biblioteca Ambrosiana and Biblioteca Trivulziana. Additionally, this study stresses the relevance of Borromeo’s officiales as key-figures to understand ecclesiastical reforms in late 16th century Milan.