BIOS is ALWAYS stored on the motherboard chipset, so changing/upgrading HDD will not affect the BIOS, unless you do BIOS reset or upgrade.

As for OEM partition, I believe you are referring to those 'hidden' or extra partitions created to store recovery images in case you need to 'factory reset' the laptop. If you are sure you don't need them or you have backed up the partition, feel free to delete it. Otherwise, just leave it there and expand your storage via other means.