The OBS – the PMI® definition
The PMI® definition for an Organizational Breakdown Structure (OBS) captures the connection between the Project Organization and the WBS, as you can see on the next page.

Organizational Breakdown Structure (OBS): A hierarchically organized depiction of the project organization arranged so as to relate work packages to the performing organizational units. (PMI® PMBOK® Guide Third Edition)

So we can be very clear about the following: the Project Organization is not an organizational chart of a company or department. The Project Organization structures the roles, accountabilities, and reporting relationships for the project, not for the company. In fact, the project organization may invert administrative relationships, with more senior persons who are accountable for individual work elements reporting to more junior persons who are accountable for higher level summary elements. The project is organized with the most appropriate person accountable for each work element, regardless of their position in the company’s organization chart. The OBS maps each person working on a project to the department in which they are employed in the company.