In Gallura, examples of Romanesque architecture are rare, yet the best-preserved basilica in all of Sardinia is found in Olbia. San Simplicio, built at the behest of the judges of Gallura, between the end of the 11th and the beginning of the 12th century, is the most impressive building in the history of Romanesque architecture on the Island. First of all, in the second half of the 11th century, the apse, pillars, columns and part of the walls and naves were built; in a second phase (first decades of the 12th century), a roof was built and the naves and perimeter walls were completed; lastly, in the mid-12th century, the façade was erected and the roof was completed.

The church, dedicated to Bishop Simplicius, who died a martyr on 15 May 304 AD, during the persecution inflicted by Emperor Diocletian, stands on a hill that was once outside the city walls.