depends on your OS.

If you are using linux, as root create a /var/spool/cron/<OracleUserName> file with the details of the job that you want to run. chown the file to root:<OracleUsersPrimaryGroup>, chmod it 0600 and restart the cron daemon (possibly /etc/rc.d/init.d/vixiecron

The error message you have given, however, does not relate to crontab directly. It is the usual error you get when you try and run an X program as a different user to the one that started X. Are you trying this in an xterm after having su'ed to the oracle user? Does the oracle user have a valid shell in /etc/passwd? does the oracle user have a valid (readable and writeable by the oracle user) home directory in /etc/passwd?

What OS? (output from uname -a)