If you can boot OOS11 you should be able to roll back to OOS10. I helped a friend to do this a few weeks ago before R was a proposition with e.
The basic procedure was to download OOS10 on pc and then transfer to the phone. (I suppose one could download direct from phone but we followed the instructions).
The file needed to be in the top level of the file manager, ie not inside a download, or other folder.This is so the phone’s local updater can find it easily.
Then it is installed with the phone local updater.