Originally the entrance wall of tomb M faced the main street, and the entrance of tomb M gave access to tombs N and O. Later tombs L and I were built against that entrance wall and the north-western wall of tomb I became the entrance of the cluster of tombs.
 At this moment, the entrance wall of tomb I is the north-  western wall of tomb M. This wall has an arcosolium in the  lower part and one niche and a small slit window in the upper  part. The side wall has also one arcosolium with a niche in the  part we still call tomb I. The rest of the side wall of tomb M  has one arcosolium and eight niches divided into three rows.
 Nowadays you can't recognize the eight niches anymore.
 Tomb M seems to have become a kind of vestibule for tombs  N and O when tombs I and L were added.

Tomb O consisted of two burial chambers above each other. The lower one could be reached by a small door (1.40 mtr in height) in the left side wall of tomb N. The vault of the lower chamber was also the floor of the upper one.
Originally the lower chamber had one arcosolium in each wall, excepting the entrance wall. These were white coloured, surrounded by red bands and decorated with flowers and leaves. Later, when more burial space was needed, three graves, separated by tuff and brick walls, were made under the floor in the middle of the room.