By default, auditing is disabled when the database is started up. You have to activate it by following these steps:
The AUDIT_TRAIL parameter must be enabled for auditing to work. The valid values for this parameter are DB, NONE, and OS:
Enables auditing to the internal data dictionary
Disables all auditing
Enables auditing to the operating system audit trail
This parameter is highly platform dependent. Consult your platform’s installation guide to determine if it is enabled for your configuration. Please refer to Chapter 12, for more information about setting INIT.ORA parameters.