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AGI I assume making an Android backup is something I shall do via TWRP before flashing the custom ROM? Would that allow me to restore the OEM software in case something goes wrong or PA does not work for me and I want to return/re-sell the phone?
I should be possible with this method. However, there's also the unbrick tool which allows you to recover the device from a total soft brick (black screen at boot). It needs Windows. I did that several times and can confirm it works. Basically it writes back every partition to the device's memory using low-level hardware access: https://forums.oneplus.com/threads/guide-mega-unbrick-guide-for-a-hard-bricked-oneplus-3.452634/
As long as TWRP still works, you could also install any OxygenOS version with TWRP, which will overwrite even TWRP.
And as long as you have fastboot access, you should be able to install OxygenOS via command line, too.
It might be useful to know which OxygenOS version you are on currently, just in case it is a very recent version that brought some firmware changes (might break TWRP or some custom ROMs' install routine).