In the Roman Catholic Church, an oratory is for all intents and purposes another word for what is commonly called a chapel. It may be private (with use restricted to an individual, such as a bishop, or group, such as a family, and their invited guests), semi-public (open under certain circumstances to the public), or public (built for the benefit of any of the faithful who wish to use it). (Code of Canon Law, canon 1223).1 The term is used for instance in the Rule of St Benedict (chapter 52) for the private communal chapel inside monasteries.
Oratories seem to have found their origin in chapels built in the sanctuaries of martyrs, for the faithful to assemble and pray on the spot. The oldest extant oratory is the Archiepiscopal Chapel in Ravenna (c. 500).