Ok, you've got a black screen. So what to do? There are many scenarios and I don't know all of them. But here is a list to review
vBIOS is unflashed or flashed with wrong vBIOS file. Easiest way to check is
Make a GRML USB boot stick ( link )
Boot iMac with GRML USB boot stick inserted, internal SATA HDD/SSD disconnected (or if you are sure its completely blank with no bootable EFI partition), and no bootable CD/DVD disc.
If screen displays GRML text screen, then vBIOS is properly flashed
Target model has to match iMac
Advanced Settings GOP Injection and GPU model has to match if necessary
If the EFI OC boots and show on screen, this is likely correct? See Boot Pickers below on "which" EFI OC
There are potentially 3 boot pickers so need to be clear what they are and how to access them
Defn here ( link ) Any bootable drive will have a small EFI partition. iMac will boot this first if the drive is selected to boot. This is where OC resides. OC is bascially OCLP's boot picker and it knows how to launch OCLP patched macOS partitions.
OCLP installer requires intalling an OC into USB's EFI partition (gets make when you say install OpenCore into the USB stick) iMac boot order is Optical Disc -> Internal SATA -> USB. So if no bootable disc and internal SATA is disconnected or is formatted blank with no EFI. Then it will boot USB EFI OC.
Similar to USB Stick EFI OC, OC can be installed into internal SATA drive's EFI. When first time installing OCLP macOS, OCLP will prompt you to install OC into the internal SATA drive. This is what it installs and eliminate a need to having to boot USB stick EFI OC followed by booting internal SATA OCLP macOS.
his is only available if VBIOS is EG version. I believe all nVidia VBIOS are EG, AMD VBIOS has no EG, EG, and EG2 (require logic board bootrom mod) Some VBIOS has no EG/EG2 versions. This boot picker can only launch non OCLP macOS partitions and EFI OC (2 differnet ones are possible per above) I believe it can not launch OCLP macOS partitions directly.
Note Option Key ONLY works with a wired Apple keyboard. Either USB keyboard or the rechargeable keyboard with the lightning to USB cable connected. BT is not enabled until OCLP macOS is running
If have option key and have valid OC EFI On both internal SATA and USB OCLP installer, you will see at last
Bootable internal SATA EFI
Bootable USB drive EFI
Any other bootable partition in the internal SATA
Note you have to boot an OC boot picker before you can boot an OCLP macOS partition. This means booting either internal SATA EFI OC or USB drive EFI OC first.
If no option key
iMac will boot internal EFI OC first. If doesn't exist, then USB EFI OC
If don't want to open the iMac again (not EG vBIOS, don't want to open and disconnect SATA drive) and have a working optical drive, can make a OC bootable rescue CD. Directions here ( link )
This occurs from time to time. And can change with newer macOS/OCLP releases. 2 work arounds has been frequently used
Reboot and resume installation from the partially installed macOS partition in internal SATA drive. This often will just resume without further problems
Same as above but hold down shift key (I think simpler boot with loading less drivers?) to get past the hang