If you extract the DD image to a USB flash drive and then boot ESXi with it, you only need to copy oem.tgz to /bootbank.
If you plan to install ESXi from CD (or USB flash drive), then you need to replace oem.tgz in 2 places
1) in the root of the CD - when ESXi runs its install process it's actually running a version of ESXi (unlike the regular ESX install which uses Linux for the install). So oem.tgz in the root of the CD (or USB flash drive) is used for the install only.
2) within the DD image within install.tgz. The DD image is written to your HD and it contains the partition which will become /bootbank when ESXi install.
See a linux script here that will create an ISO image for you with a custom oem.tgz file - viewtopic.php?f=7&t=4