The Collegiate Church of Sant’Orso stands above the cruciform Paleochristian Church of San Lorenzo, its predecessor. The Romanesque church as we know it today began construction in the 9th century and currently incorporates the bell tower, which originally formed part of a defensive wall, and the cloister.

The interior of the building consists of three aisles ending in semicircular apses. Of remarkable artistic value is the wooden choir, the cycle of 11th-century frescoes, and a square mosaic floor composed of white, black, and brown tiles. The main depiction in the mosaic shows Samson slaying the lion.

The jewel of the monumental complex of Sant’Orso is the cloister dating back to 1132, created by craftsmen from Provence and/or Lombardy. The form we know today was shaped by the imposition of Pope Innocent II, who introduced the rule of St. Augustine to the canons of Sant’Orso. The previously carved marble capitals, coated with black paint, were used as bases for high reliefs depicting the parables of the Old and New Testaments, the life of St. Orso, as well as various mythical creatures and decorative elements. It is considered a masterpiece of Romanesque sculpture. Within the cloister, there is also a beautiful centuries-old lime tree.