Success - 15.1 OCLP on Retina 5K, 27-inch, Late 2015 iMac

https://preview.redd.it/w9bxk6doptxd1.png?width=280&format=png&auto=webp&s=b8eb5f1e51b3573ba1db16fc9bc53867c275c12a

Played it safe, and installed via macOS installer upgrade booting from external USB.

Note that I have a LAN connection, as Wifi will stop working without patching the System Volume.

Once installation completes, run via ssh:

sudo /Applications/OpenCore-Patcher.app/Contents/MacOS/OpenCore-Patcher --patch_sys_vol

sudo reboot

Then login as normal, and build and install OpenCore to the internal drive.

Job done.