Olbia is an ancient city, probably Punic despite the tradition that the Greeks in the West used it to frequent it. The church is located in an elevated position and in correspondence with the ancient western Roman necropolis affected by continuity of attendance including, at the current state of research, between the 7th century BC, as a cultural use and the 3rd century BC and the 4th century AD as a specifically funerary use.
The structure of the church and its clients, in the current state of research, are not known at all. The church has been archaeologically studied and in relation to masonry techniques it has revealed 5 construction phases, between a period prior to the eleventh century and the second half of the thirteenth century.
Originally, the smaller basilica extended up to the fourth pair of internal arches with a roof made entirely of wood. Problems related to the roof made it necessary to close the side aisles with a 'barrel' vault, using bricks arranged lengthwise. By creating empty spaces to cover the naves, an insulating layer above the apse and building on these layers using new granite, the Basilica was raised to its current height. Later, the church will be extended by two pairs of arches and will see the second façade in the last pair of internal pillars. The preparation of a never completed bell tower, near the structure, prompted the workers to advance the construction by a further pair of arches, incorporating the tower.
The church (33 x 13 m, about 12 m high) has a plan with three naves divided by arches on pillars and columns, according to an alternate system of supports. The capitals are also made of granite. Crowning the pillars are thick pyramid-shaped abacus. Among the capitals, one has rounded corners, attested to the Po Valley starting from the Early Middle Ages, while the decorated ones have the general shape of an inverted trunk cone and two others are simply column bases used for this purpose. Two of these capitals would be represented respectively: one faces surrounded by birds and the other of ram protomes, motifs framed in the Longobard era.
The central nave, covered by a wooden roof, ends with the WNW apse. The façade is divided into two orders. In the lower one, there are three mirrors. In the inner field of a bow, shaped like a marble panel depicts a scene with a knight and a long gun set against an angel with a knife, a scene stylistically traced back to the 7th century. In the central mirror, the arched portal opens with an exhaust arc, off axis with respect to the three-light window of the upper order and still off axis with respect to the double slope of the tympanum. Inserted in the recessed field of a blind arch, the three-light window consists of two columns, one with an ophytic knot. Eight ceramic basins complete the decoration. (text update: Dr. D. Bacciu)
Now the Basilica is home to the Civitatense Museum, part of the Diocesan Museum System under the protection of the Cultural Heritage Office of the Diocese of Tempio - Ampurias. Info +39 345 6328150 - museumcivitatense@gmail.com

History of studies
Reported since the 19th century by Giovanni Spano, the church was also the subject of considerable attention during the 20th century. They were interested in the historical and artistic events of the Dionigi Scano monument, Raffaello Delogu, Renata Serra, Roberto Coroneo and Anna Pistuddi and finally Antonio Careddu and Marcello Cabriolu.

Bibliography
G. Spano, "Antica città di Olbia, e sua cattedrale", in Bullettino Archeologico Sardo, VI, 1860, pp. 145-149, 173-174;
D. Scano, Storia dell'arte in Sardegna dal XI al XIV secolo, Cagliari-Sassari, Montorsi, 1907, pp. 124-128;
R. Delogu, L'architettura del Medioevo in Sardegna, Roma, La Libreria dello Stato, 1953, pp. 92-95;
R. Serra, La Sardegna, collana "Italia Romanica", Milano, Jaca Book, 1989, pp. 322-329;
R. Coroneo, Architettura romanica dalla metà del Mille al primo '300, Nuoro, Ilisso, 1993, scheda 14;
A. Pistuddi, "La chiesa di San Simplicio ad Olbia (SS): contributo allo studio dei capitelli", in Annali della Facoltà di Lettere e Filosofia dell'Università di Cagliari, n.s. XXI (vol. LVIII) - 2003, Cagliari, 2004, pp. 155-173;
R. Coroneo-R. Serra, Sardegna preromanica e romanica, collana "Patrimonio artistico italiano", Milano, Jaca Book, 2004, pp. 111-122;
R. Coroneo, Chiese romaniche della Sardegna. Itinerari turistico culturali, Cagliari, AV, 2005, p. 65.
M. Cabriolu, Basilica minore di San Simplicio, Università degli Studi di Sassari, Tesi di laurea in Scienze dei Beni Culturali curriculum archeologico, AA 2018/2019;
A.M. Careddu, Arte e manufatti artistici nella Sardegna medievale. San Simplicio di Olbia, tesi di laurea in Lettere Moderne AA 2006/2007, Università degli Studi di Sassari;
Bacciu D. - Cabriolu M., San Simplicio in Olbia: l'indagine attraverso la lettura delle strutture murarie, in VI ciclo di studi medievali, atti del convegno 8-9 giugno 2020 Firenze, a cura di NUME gruppo di ricerca sul Medioevo latino. Lesmo, Etabeta, 2020, pp. 4-10.