@Moem yes, I did. But the very first question I cannot answer is: should I use the full system image or halium-boot method or the stand-alone kernel method. I don't understand the differences. As I mentioned: I'm not very familiar with mobiles.

Wow .. that really work out! Thanks to @MrT10001 !! I had to change one line in the install script of OxygenOS, because my device had forgotten that it is a OnePlus. Then it worked. Ubuntu Touch is running now. Have some miner "issues" like I don't know how to close camera app .. but I will figure it out. Best DatPhil