I found the following Database Architecture Diagram for Oracle Database 12c on Oracle Education's website. It lists all the processes and the relationship between processes and other database components. I recommend you to take a print on A3 paper.
And below is a YouTube link which gives database architecture overview.
Oracle Database 12c Architecture Overview
Update - 1 (11-Jul-2013):
Below is Oracle Database 12c Multi-Tenant Architecture Diagram: