You need the equivalent of EL5.7 or 5.8 in order to have an installer that supports the newest generation of Smart Array controller.
Please see the footnotes on the HP/OEL technical exceptions matrix.
You need Oracle Linux/UEK 5.8. That's the minimum-supported installer version.
What version is your OEL installer?
This was an issue with multiple elements. First, the OS the original poster was installing was too old for the server hardware. That was resolved by viewing the technical exceptions matrix from HP for Oracle Enterprise Linux. The same applied for RHEL and CentOS.
Second, I assumed the OP was using a proper Smart Array RAID controller. The P420i is the right controller for this setup. Instead, he is using a "Dynamic Smart Array B320i", which is a fakeraid controller. See these notes from the CentOS forum. It uses a software-side closed-source driver to enable RAID functionality. Terrible.
Red Hat explains that the servers with this controller are shipped in a bad default configuration that will prevent installation.
Resolution: Systems with these controllers will ship from HP with the Software RAID mode enabled by default. You can either load a driver disk/USB key at installation for your OS or disable the RAID...
This mode requires a closed source driver that is not shipped or supported by Red Hat.
In order to install RHEL on these systems without the closed source driver and RAID support, the Software RAID mode must either be disabled (B320i controller) or switched to SATA AHCI support mode (B120i SATA controller).
For systems with the B320i SAS controller:
1. Boot the server into System Options
2. Navigate to HP Smart Array B320i Raid Configuration
3. Change to DISABLED