Right, because otherwise there is hot air constantly infiltrating the space between floor and ceiling, right? Also, all insulation needs an air barrier, right? Otherwise it just becomes an air filter instead of insulation. So, the space would need blocking/plywood/drywall or some other material, correct?

In regards to the presumption that the OSB wall had insulation, its not visible, it would be behind the OSB, so I don't know.

What about the exterior walls within the truss space between floors. I am assuming that needs insulation also, correct?