At least seven readings have been proposed for the name Osoppo: the Latin Os-Apertum, the open place, Os-Oppidum, the fortified place, Os-Opum, the place of riches, Hyssopum, from the hyssop plant, and Aus-opum, the place by the river; the pre-Roman Os-Osis, from the ash tree; and the Gallo-Carnic Os-Hop, the place for calling aloud. Whichever is the right one, they all describe the same thing: a hill from which you can see a long way. That hill controlled the routes joining the plain to Carnia and to the Alpine passes, and it mattered before the Romans came, when the Carni lived here, a Celtic people whose words and sounds Friulian still keeps. The first documentary records are of the eleventh century, when the castle belonged to the Benedictine abbey of Santa Maria in Silvis at Sesto al Reghena.
Towards the middle of the thirteenth century the patriarchate of Aquileia granted the castle in fief to Cono, son of Amaldarico. His descendants went their own way, quarrelled with their neighbours and drew close to the counts of Gorizia, the chief rivals of the patriarchate; and rather than let such a stronghold slip into the Gorizian orbit, in 1328 the patriarch took it from them and invested Federico Savorgnan with the fief. When Venice conquered the patriarchate in 1420, the Republic understood at once what that hill was: the fortifications were enlarged and modernised, and Osoppo became the chief bulwark of the north-eastern frontier of the Stato da Terra. In 1514, in the war between Venice and the Empire, the fortress was in the hands of Girolamo Savorgnan, who refused to surrender to the imperial troops. The summit of the fort, at about 330 metres, is still the highest point of the comune; on the slopes of the Colle di Napoleone, from Venetian times until the Second World War, there was a barracks. Around it lies the wood, with its ring of paths, the springs of Bârs and the orchids that grow there of their own accord.