Organizational structures represent the enterprise grouping of resources to perform work. This work can be project or non-project related.
Once the WBS and OBS are developed the two structures can be intersected to generate a responsibility matrix table. Each Work Package or activity will have a WBS code and an OBS code indicating who is responsible for the delivery of what package.
For most projects it is recommended that the first level of the WBS be time phased oriented typified by your organization’s system development methodology (Definition, Analysis, Design, Development, Testing, Installation, Training etc).
Per type and size of the project, each phase can be decomposed in to more detail into smaller and more manageable components easy enough to assign clear responsibility for completing the task.