The Building with Opus Sectile was built 100 meters outside the Marine Gate, directly behind the ancient beach. A modern road is more or less on top of the ancient coastline. The building was partially excavated in the years 1938-1942, but Guido Calza and Italo Gismondi stopped their excavation after discovering the famous marble wall decoration, opus sectile, after which the building was named. The opus sectile was unearthed further in the years 1959-1961. The building was published in detail by Giovanni Becatti in 1969.

From 83 coins can be deduced that the construction started in the years 385-388 AD or a little later (opus vittatum), but some Hadrianic masonry (opus mixtum) can still be seen. The unfinished building was destroyed in 393 AD or somewhat later. Traces of fire were not found. The cause may have been an earthquake (note that in the years 393-394 AD the cella of the Temple of Hercules (I,XV,5) was restored). Becatti suggests that the destruction was the work of people, during a well-known pagan revolt under Eugenius. Afterwards work did not continue, the building was never used.

The building was entered through a vestibule with a porch (A) at the end of the Decumanus Maximus. In the entrance is a threshold with pivot-holes for doors. The vestibule leads to the north-east corner of a colonnaded courtyard (B), that for the most part was destroyed by the sea and is covered by the modern road. Remains of a west-east running dike and of a mole perpendicular to the dike were found, perhaps built in the first century AD (Claudian?). The dike was given up during the late-antique construction work. To the east of the courtyard is a row of rooms, including an exedra (C), with two columns in the entrance and with a marble floor. Further rooms were found to the north of the courtyard, including a large staircase towards the west (not on the plan) and room D, where the opus sectile was found that gave the building its name. For some reason the level in this part of the building was lowered 1.5 m. In the entrance of room D are two columns, resting on the abandoned dike. The back part of the room is an alcove or exedra. There is a door in the east wall.

The opus sectile was found on the floor and restoration took many years. In 2006 it was taken from the museum in Ostia to the Museo dell'Alto Medioevo in Rome (EUR). If you want to see it back in place, then have a look at this reconstruction movie. Only the upper part of the marble decoration of the walls had been finished when the building was destroyed, from 2.50 to almost 8 meters from the floor. Work on the lower part had not yet begun. In the front part of the room are three registers on the side walls. On the east wall were depictions of animals and people. There are two lions (yellow) attacking a deer (blue-grey), a bust of a blessing Christ with nimbus and beard, and a bust of a young man. On the west wall was a tiger attacking a deer. Are the wild animals and the deer a reference to the amphitheatre? Paul Zanker has suggested that the head with nimbus is that of a philosopher, but that view has won little support. There are furthermore depictions of architecture and of a window.

The south side of the side walls of the back part of the room were decorated with pilasters in opus sectile, with depictions of vine tendrils, birds, snails, butterflies and worms. On the walls of the back part the marble imitates opus mixtum and windows, and even shadows. The imitation of the brick-and-tufa opus mixtum is surprising: it is material that is of course inferior to marble, and this particular combination had at the end of the fourth century not been used for centuries in Ostia (but some Hadrianic opus mixtum was still visible in this building). On the lower part of the wall is a checkerboard pattern, perhaps tapestry or opus reticulatum.

The ceiling seems to have been decorated with mosaics with a blue background. There are no parallels for mosaic decoration of ceilings, which was a late-fourth century invention as we can read in a letter written by Symmachus: Novum quippe musivi genus ... in tegulis (Epist. VIII, 42). Above the entrance to the exedra was an architrave with depictions of furniture and other objects. No pavement was found, but a number of marble slabs intended for its decoration were already in the building.

Was the room with the opus sectile a dining room, or did it have a religious function? According to Becatti the building was the seat of a Christian guild, but the presence of such a wealthy guild in Ostia at such a late date is surprising. It has also been suggested that it was a seaside villa. It may also have been the Ostian office-residence of an official, such as the Praefectus Annonae, and perhaps it was the work of Ragonius Vincentius Celsus, who was very active in Ostia while he was Praefectus Annonae, in the years 385-388 AD.