Here are some useful links for Oracle Certification Online Pratice Quiz and Tutorials
You can practice, evaluate your Knowledge on various
Oracle Cerfication Exams and get assessed for FREE.
Quiz Questions are generated randomly.

Oracle Database Skills Practice Test

Oracle8i: New Features for Adm Practice Test

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Oracle 9i Tutorials
Oracle 10g DBA Tutorials
Oracle Apps 11i Tutorials

Hope these helps you...