Hi Samira,
For complete info on Oracle Certification track please visit Oracle Certification Homepage (www.oracle.com/education/certification/ ). Please do let me know if anything else I can do for you.
As of now, one can be certified DBA in 8i or 9i. For 8i, you have to write 5 exam and then one upgrade exam to gain 9i certification. Whereas there are 4 exams for 9i and its in two parts, clear the first two exam and become OCA, pass the next two exam and becom OCP.
Each exam cost US$125 (register at otn.oracle.com and get 20% discount, cost per exam will come down to US$100).
Time??? it depends, I was out of job and was studying full time so I managed to get it done in 3.5 months, but again it depends person to person and situation to situation.
I used Sybex study guide (three books in that kit).
STS (
www.selftestsoftware.com )is the commercial grade mock exam which costs US$ 80- US$ 100 per exam but very useful.
Other sites for OCP are
www.dbasupport.com ,
www.orafans.com Additionaly you can also visit Oracle/OAS forum here at javaranch to know more from others about oracle issues.
HIH,