It has an area of 6,250 sq. km., almost identical to that of the civil province, with a population of 578,420 inhabitants, of whom 578,377 are Catholics distributed in 602 parishes, served by 516 diocesan priests and 36 regular clergy; it has two seminaries, 9 male religious communities and 24 female ones (Ann. Pont., 1952, p. 302).
It was first a suffragan of Braga and then of Santiago de Compostela. The first known bishop of O. is Witimiro, present at the Synod of Braga in 561. Occupied by the Arabs (732) and reconquered by Alfonso III (c. 877), the diocese revived only to be suppressed c. 977 and incorporated into Lugo, due to the Norman invasions. It was re-established on 13 January 1071. Among the illustrious bishops of O. are to be noted St Ansurius in the 10th century and John of Torquemada in the 15th century. The patron saint of the diocese is St Martin. The cathedral, begun in the 12th century and completed in modern times, is a heterogeneous construction in which every century has left its mark.