What is an Organizational Breakdown Structure (OBS)?
The OBS is the hierarchical chart of the project organization — departments, teams, units — arranged so work can be assigned to the parts of the organization that perform it. Cross it with the WBS and you get the responsibility assignment matrix: WBS says what, OBS says who, the intersection says who-does-what.
Control accounts typically live at those intersections — one piece of scope, one organizational owner, one budget — which is what makes performance attributable.
Worked example
A shipbuilder's OBS branches: hull production, outfitting, electrical, commissioning. Crossed with the WBS, element 4.2 "navigation suite" lands squarely in electrical's box as a control account with its own budget and manager. When 4.2 runs over, nobody debates ownership across three departments — the OBS intersection settled that before the first weld.