In reply to others in this thread and, I hope, to give some useful help to anyone planning this upgrade to Android “R” on the OP6, I thought I would give a longish description of how I managed it. Having similarly upgraded two other phones - one a OP5T - I found this upgrade the longest and most complicated. However, this may be due to my uncertainties and lack of expert knowledge as I was most concerned by the warning on both the Lineage site and here on the upgrade page for this device:
before flashing /e/R install Stock Android 11
As I don’t use Windows and wanted to avoid the hassle of using the MSMDownload Tool to completely restore my phone to a stock ROM version of “R”, I followed the instructions above from @kopap - many thanks to himfor this info.
This requires unpacking the necessary *.img files from a recent OxygenOS ROM file. To get these files at the suggestion of @Tentos I downloaded it from the German OnePlus site.
However, from my understanding the “22.J.52_OTA” version still comes with Android “Q”, which left me a bit concerned as it is the 22.J.62 version which comes with “R”.
As my PC has Arch Linux I was able to use the excellent AUR version of payload-dumper programmed with the “go” language instead of python and it extracted the files needed for the flashing listed by @kopap above. Using ADB I put my phone in fastboot mode to complete the procedure with no difficulty encountered.
Rebooting into Recovery I expected there to be an option to complete the upgrade to version “R” of OxygenOS but I was back in the e-recovery installed by previous updates. Beforehand I had copied the OTA zip file 22.J.62 to flash this if necessary to have the required firmware for the upgrade. However, I could find no way to use e-recovery to flash this from internal memory.
Crossing my fingers I decided to go ahead with sideloading the most recent e-os-r dev file for enchilada and see what happened. I did a Factory reset/ then Format data/factory reset before the sideload on the active slot_b but avoided sideloading the copy-partitions zip file in case the upgrade failed so I could switch to slot_a and be back with my previous “Q” version.
To my surprise, after the sideload installation completed without a hitch, my phone rebooted into a clean version of e-os under Android “R” even quicker than with the upgrades of my two other devices.
To be able to root my phone and use my Titanium backup to restore my previous apps & data I decided to install the latest TWRP by booting the twrp.img file first and then following the recommended procedure to have this recovery installed in the Ramdisk permanently, also in case I need to re-flash the copy-partitions file to have both slots on the same version. In my thinking the next update should do this automatically, but I may be wrong and would be grateful if some more knowledgeable person puts me right about this.
I am still left perplexed as to why the flashing of the above-listed *.img files should have worked especially as the boot.img and the system.img files extracted with the others were not included. To have a working stock OxygenOS I would have thought they at least would be necessary, but it explains I suppose why my reboot into recovery landed me in the initial e-recovery.
Thanks to all in this thread for helping me with this not-so-simple upgrade to e-os-r