To pay attention to the following aspects
1. Modify the sys, system password.
2. Lock, modify, delete the default user: dbsnmp, ctxsys and so on.
3. The REMOTE_OS_AUTHENT into False, to prevent remote machines direct landing.
4. The O7_DICTIONARY_ACCESSIBILITY into False.
5. cancel some privileges from PUBLIC Role.
6. The safety data file to check the database. Do not set to 666 and the like. Check other dba user.
7. some unneeded services (such as ftp, nfs, etc. shut off)
8. number is restricted host database above.
9. regularly check Metalink / OTN above security Alert. For example: http: //otn.oracle.com/deploy/security/alerts.htm
10. Put your database and application in a separate subnet, or else your user password is easy to go sniffer. Or with advance security, encryption of user login.
11. limiting, only certain ip to access your database.
12. lsnrctl to add password, or else it easy for someone to turn off your listener from outside.
13. If possible, do not use the default port 1521