[oracle] sqlplus error on select from external table: ORA-29913: error in executing ODCIEXTTABLEOPEN callout

Keep in mind that it's the user that is running the oracle database that must have write permissions to the /defaultdir directory, not the user logged into oracle. Typically you're running the database as the user "Oracle". It's not the same user (necessarily) that you created the external table with.

Check your directory permissions, too.