The multi-org Structure of Oracle Apps is
Business Group (Top most level) to which employees are assigned which segregates the Oracle HR data
Legal Entity (2nd level) is the organization level at which financial and tax reporting occurs. A legal entity links to a GL Set of Books.
Operating Unit (3rd level) is the subdivision of an organization within a GL Set of Books. Other Oracle Apps subsystems like Receivables Payables etc. are defined at the Operating unit level. Security may be defined at this level to disallow one Operating unit from entering or viewing data of another Operating Unit.
Inventory Organization (last level) is required for an organization that utilizes Inventory functionlity. The inventory org typically represents different inventory locations such as a warehouse.