So I'm 60% of the way to my 9i OCP now. I passed 1ZO-031 today. I already had the 1ZO-007 test and a hands-on class under my belt.
So far I've been using the Certification Candidate Guide, Oracle Press books and of course the Oracle documentation. For my next two exams I got some Sybex books cheap off of Amazon.com that I'll be checking out.