The patron saint of Olbia and of all Gallura is Saint Simplicius, celebrated on 15 May with religious celebrations beginning nine days before. The saint, under the reign of Emperor Diocletian, was pierced by a lance and died after three days of agony together with his three companions Rosola, Diocleziano and Fiorenzo, and was buried in the Roman necropolis outside the city walls. In that same necropolis, during excavations in 1904, a treasure was discovered containing 871 gold coins bearing the mark of 117 different Roman families.
It was precisely on that necropolis that, between the late 11th and early 12th centuries, judge Constantine III of Gallura had the extra-mural basilica of San Simplicio built, still today the region's most distinguished Romanesque-Pisan monument: built entirely of granite blocks by Tuscan and Lombard labourers, it was for centuries the city's ancient cathedral. Olbia was indeed the ancient capital of the Giudicato of Gallura and the region's first episcopal seat, part of the diocese of Civita-Ampurias until 1839.