Cesare Baronio, C.O. was an Italian Oratorian, cardinal and historian of the Catholic Church. His best-known works are his Annales Ecclesiastici, which appeared in 12 folio volumes (1588–1607). He is under consideration for sainthood and, in 1845, Pope Benedict XIV declared him Venerable.

The Confederation of Oratories of Saint Philip Neri, abbreviated C.O. and commonly known as the Oratorians, is a Catholic society of apostolic life of pontifical right for men who live together in a community bound together by no formal vows but only with the bond of charity.