I had the same problem and analysed it deeply. The problem is that Macbook has too old firmware to boot from the OWC drive. If you run High Sierra or above (installer or installed system on external disk or recovery image), it will recognise the disk correctly. Upgrading firmware on Macbook however, required that it is first installed on the internal drive on a special EFI partition, and then system reboots, and it upgrades firmware during boot. Of course it will not upgrade the firmware, because after reboot it will not see the internal drive. So this is a chicken-egg problem.
The solution is to install original Apple internal drive (or another drive that works with old firmware) and to update firmware either by installing High Sierra or newer OS or manually using “bless” command. Then the Apple drive can be removed and OWC drive installed and it should work fine. OWC claims it has a drive rental service, but I haven’t checked it. I bought a 128GB used Apple drive for the firmware upgrade.