The archaeological dig carried out in several batches between 1976 and 1999, allowed for the building’s constructive stages to be rediscovered. The digs involved an area which used to be part of a large extra-urban necropolis, where, at the beginning of the 5th century, there was an early-Christian complex which also included the cruciform church of S. Lorenzo. At the centre of the south nave, the basement of a funeral building was found, it may be dated to some time between the 4th and 5th centuries A.D.; the primitive church, which was erected to the north of this mausoleum, consisted of a simple apsidal hall surrounded by a portico destined for use as privileged burial grounds.
In the 9th century, the church was completely rebuilt and enlarged, moving the general axis of the building southwards, the eastern extremity has three apses, while the facade was rebuilt to the west of the early-Christian one. In the year 989, a bell tower was added to the facade, the remains of which are still visible up to a height of approximately 15 m.
The archeological dig of the choir of the church of S. Orso allowed for a square-shaped floor mosaic to be brought back to the surface, it was unknown and not mentioned by the sources, it was made with black and white tiles with some inserts of light brown coloured tiles. A series of six circles inscribed in the square, acts as a frame for the central decorations. In the central medallion there is an elegant representation of Samson killing the lion.