You can simply write an OEM image to the "Kernel" partition using mtd.
Or you can write it to the "Kernel2" partition and keep OpenWrt in "Kernel" if you like. Then you can switch to OEM from OpenWrt using
fw_setenv BootingFlag 1
reboot
and from OEM to OpenWrt using
nvram setro uboot BootingFlag 0
nvram commit
reboot