I just answered a similar question this week. You can see the answer here.
However, I would caution that dumping data to a flat file is not a backup strategy that I would employ. I just cannot image a scenario where comma-delimited files would play a role in my backup and recovery strategy of my Oracle database. How do you recover the contents of your control file? How do you recover changes to that data since that backup was made? How do you recover your indexes, sequences, stored procedures and other database objects? You cannot recover any of these with a comma-delimited file.
This was first published in January 2006