Because the OBS is maintained as a separate, global hierarchy, you have flexibility in making your OBS assignments as responsibilities change throughout a project life cycle.
You might choose to create your OBS to match each EPS node and project set up in the EPS. You could initially match OBS names to the EPS node and project names. You could then assign users, by their login names, to the OBS elements to grant access to the corresponding EPS nodes or projects.
The type of access granted to a user is determined by the project security profile assigned to the user.