OS_ROLES determines whether Oracle or the operating system identifies and manages the roles of each username.
TRUE: the operating system completely manages the role grants for all database usernames. When a user attempts to create a session, the username's security domain is initialized using the roles identified by the operating system. A user can subsequently enable as many roles identified by the operating system as specified by the parameter MAX_ENABLED_ROLES.
Revocation by Oracle of roles granted by the operating system are ignored, as are any roles previously granted by Oracle.