Steps:

1. run $ORACLE_HOME/bin/localconfig delete
2. rm -rf $ORACLE_BASE/*
3. rm -f /etc/oraInst.loc /etc/oratab
4. rm -rf /etc/oracle
5. rm -f /etc/inittab.cssd
6. rm -f /usr/local/bin/coraenv  /usr/local/bin/dbhome  /usr/local/bin/oraenv

7.remove the oracle user and the related group(dba,oinstall)

Notes:These are the simple steps,I will research and explain why we need to do these steps.A detailed instruction would be post later.