Modern-day Oderzo, in antiquity Opitergium, was the administrative centre of Byzantine Venetia from after the invasion of the Huns in 452, until during the Lombard conquest, around 640.
Following the Lombard invasion and conquest of Venetia, a part of the Venetian population of the mainland cities fled or left for the settlements in the nearby lagoons.
In late Antiquity the north-east of the Italian peninsula suffered many invasions. The wars caused people to move to the safer lagoon areas, and hence to the foundation of Venice.