Sant'Orso had originally a single hall, delimited by a semicircular apse. It was entirely rebuilt during the 9th century, during the Carolingian age. Later, bishop Anselm of Aosta further renovated the church, introducing a basilica plan with three naves with wooden trusses. These were replaced by Gothic cross vaults in the 15th century.
The church has a nave and two aisles divided by quadrangular pillars.
The vault was rebuilt in the 15th century. Fragments of a Romanesque series of paintings are preserved in good condition in the space between the current vault and the original ceiling. These portray scenes from the New Testament as well as a martyrdom. Stylistically they resemble the bright colours and strongly marked outlines of some of the frescoes at the Galliano Basilica near Cantù. In the right aisles is a chapel houseing the altar of St. Sebastian, also with frescoes (15th century).
The cloister has historiated capitals depicting the life of Ursus. 37 of the 42 original capital remains: they were originally in white marble, though now they mostly appear in dark gray color aftery they were washed with ash paint.
The quadrangular-plan bell tower, dating to 989, has kept some the lower 15 metres of the original medieval structure. The present structure, in Romanesque style, dates to the 12th century, and has a total height of 44 metres.
The church is home to numerous missals and reliquaries, including the relics of Ursus, which rest in the crypt. It also holds the relics of Saint Gratus of Aosta.
References:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.