@Edhawk
Yes is there is no logic in this, but I found a work around.
There was nothing wrong with the CMOS battery, but I replaced it anyway. Original still held a voltage of 3.23V. New one was 3.3V. The only way I could get the BIOS to see the UEFI USB Opencore stick was to re-flash the BIOS and load the defaults, set some defaults and reboot. When I did that the BIOS sees all the drive / USB boot options available to it.
This is a Gigabyte Z97x motherboard. After one successful boot to OSX the USB stick was no longer seen by the BIOS. So one time I changed the BIOS boot option to UEFI Only boot, from Legacy and UEFI boot mode. The BIOS now saw the USB Opencore stick. But my dual boot, one drive OSX the other Windows, now only saw the USB stick. Eventually I was able to mount the EFI partition of the NVME drive on the motherboard. I was using Opencore Configurator to mount the EFI partition. The Configurator offers the choice of either the APFS partition of the NVME or the 'root' partition. I choose the 'root' EFI partition and copied the USB stick EFI to it.
I was never sure which of those two options to choose from the Configuator but I stuck to the 'root' EFI partition to mount - possibly mounting either results in the same EFI partition being mounted??
And following the copy of the USB EFI to the NVME driver EFI partition I was able to reboot using the SSD, with the USB removed from the system.
After a NVRAM reset, and a copy of a Microsoft boot folder to the EFI folder of the NVME EFI partition, I was able to see the second drive containing Windows in the Opencore boot selector.
To cut an even longer story shorter, I was able to boot into Windows, iff I turned off the iGPU in the BIOS. But OSX wants the IGPU enabled. Go figure.
And finally I found that the Z97x motherboard supports a dual BIOS, but I can only get the 'slave' BIOS to work. Setting the motherboard BIOS to the 'master' or #1 ROM just gives me a splash screen, no keys working, frozen system.
A cross between a HW issue of the ROM and a transition version of the BIOS (F8) that doesn't do mixed UEFI and legacy booting correctly (not that I care about Legacy booting).