What’s amazing is, most of Ostia was just abandoned at the end of the Empire (i.e., when the city of Rome fell) and apparently never occupied again.
Just because thousands of well-armed barbarians with really ugly reputations can be seen approaching by the dust cloud, everyone in Ostia (and probably most of the Romans) decided to hit the road, or in the case of those who had somewhere to land, take to ships.
The buildings of Ostia slowly got partially buried by blown in sand and whatnot, plus the decomposition of various wood construction elements. The whole place was just sticking out, everyone know what it was and where it was, and no one did anything much with it until, I think, Mussolini.
According to a historical novel about Rome I am currently reading, when the Gauls came in, most people fled to a nearby town they had recently conquered (Veii I think it was). The Gauls managed to burn and destroy a lot of Rome, and after they left it was decided to rebuild Rome and take most of the building materials from the other town’s buildings.