You can setup a batch file that can run a schedule to run @ night

NOTE: this is using Windows

first run an sql file that will truncate the tables you want to delete data out of.
the run the sqlldr to import the data into Oracle.

Example batchfile

@Echo off
REM truncate tables
sqlplus scott/tiger @c:\temp\truncate.sql
REM bulkload data into Oracle
sqlplus userid=scott/tiger contol=d:\temp\test.ctl log=d:\temp\log\test.log

Within the body of the "truncate.sql"

spool c:\temp\log.out
truncate table temp1
truncate table temp2
truncate table temp3
quit

hope this helps
scotta