Apple diagnostics will not open by holding D or 'option D' on startup so I'm not sure I can answer that, maybe it has something to do with the clover bootloader? My specs are:
Motherboard: asrock h110m-itx a/c
GPU: NVIDIA GeForce GTX 760 2047 MB graphics
CPU: 3.7 GHz Intel Core i3 SkyLake
From what I've read these parts are all quite compatible with Mac OS X. The GTX 760 was specifically recommended as a GPU that is compatible with OS X so I am hesitant to accept that the GPU is at fault. The motherboard however is a fairly cheap itx so maybe it is just a driver (or kexts as they're called on mac?) issue.
I may try and upgrade my motherboard and retry Opera at a later date.