Formerly inhabited by Celtic populations, Osoppo acquired a certain importance at the time of the foundation of Aquileia, becoming a Roman castrum; as evidence of this, some archaeological excavations have brought to light several finds, among which stands out a tomb of a leader dating back to the 1st century BC.
The main monuments of the town were also seriously damaged by the earthquake of 1976, however many architectural works have been expertly restored and can now be admired again. The hill and the Fortress of Osoppo represent a complex of great historical and naturalistic interest. The hill has been the scene of some glorious episodes and has been used since ancient times as a military structure continually adapted to the modification of military strategies and techniques: today you can visit tunnels, ditches, trenches and casemates; for these reasons in 1923 the Fortress of Osoppo was declared a "national monument". In the southern part of the hill, fossil footprints of mammals that lived between 2 and 10 million years ago are visible.