Your server uses UEFI-compliant server firmware that supports UEFI and standard BIOS and should detect the correct mode. However, it favors UEFI over BIOS. http://www-947.ibm.com/support/entry/portal/docdisplay?brand=5000008&lndocid=MIGR-5083207
Keep in mind, that any disk that is larger than 2 TB requries the GPT partitioning scheme. The GRUB bootloader of Oracle Enterprise Linux 5.5 does not support GPT or EFI. I suggest to check the product documenation and installation notes specific for your system.