I will not try to give detailed advice, I do not have the phone, I have a OnePlus One from when things were simpler ! In my case I have the ROM and I can fastboot flash all (that is all partitons). Please treat a rollback from Android 13 as a specialist task and read carefully !
I suggest you study the advice given in the kebab thread I already linked.
If you find instructions you fully understand to use a fastboot flash method to rollback this is the nicest way to go (I have no bookmark of such method for this device). I did not fully investigate the advice here OnePlus 8T Touchscreen Doesn't Work After Install - #7 by descrato
My current knowledge does not extend beyond the advice I gave here OnePlus 8T Touchscreen Doesn't Work After Install - #9 by aibd.
Please be especially careful to understand this point OnePlus 8T Touchscreen Doesn't Work After Install - #15 by RdLr – your method of rollback must not leave any traces of Android 13 (or Lineage 20) in any partition.
Once you have done the rollback successfully, I would be inclined to give the phone an extended test on Oxygen maybe for a day or too – maybe over cautious !
Good luck
Update.
It is very cheeky and unorthodox to call for /e/ support in this way but, @marcdw, might you have a link to an Oxygen “Fastboot” ROM for OP8T which would be an improvement on what I have written so far ? Thank you. 