Hi,
Yes u cannot audit DROP TABLE.This is because when you create a table you are the owner of the table and so you have all the priveleges.If you want to audit CREATE,DROP and other DDL commands write a database trigger on it.The database trigger works fine on Oracle 8.1.5 and higher versions.
In case of any help please be free to ask me at [email protected]

Regards,
Rohit Nirkhe ,Oracle DBA,OCP 8i
[email protected]