Read this thread.
Before starting ensure the device is charged to over 80%. It uses a lot of power setting up.
Download the rollback build OB25 (OnePlus6Oxygen_22_OTA_054_all_1908302047_fulldowngrade_f019784c78234a67.zip) from the link here
Enable developer options on the device by going into Settings, About Device and tapping build number till it tells you have Developer Options enabled.
Go to Settings, System and Developer Options. Enable USB Debugging.
Connect the device to the PC.
A prompt will appear on the device, so accept it.
Draw down the top of the screen of the device from the left side and you will see a Dialogue box with a cog that states something like Android Actions. Expand that and choose File Transfer.
On the PC copy the downloaded OB25 to the root of the device, i.e. do not put it in a folder.
Once coped over, disconnect the device and go to Settings and System Updates. In the cog at the top right of the screen choose Local Upgrade.
You should see the zip file you have transferred over. Tap on that and start the downgrade. It will ask to reboot at 100%, reboot and let it do it's thing.
Set up Android again, ensure you connect to Wifi and you are in Android 9. Enable Developer Options again and enable OEM Unlocking (this will be greyed out if you aren't connected to Wi-Fi).
Reboot to fastboot mode and Unlock the Bootloader - this involves having the ADB and Fastboot drivers installed on the PC and opening a terminal in the folder that contains them.
Run the command ./fastboot OEM unlock
Follow the instructions on the device, once the bootloader is unlocked, the device will reboot.
Set up Android again. Enable Developer Options again. Enable USB Debugging again.
Start up the Ubuntu Touch installer and connect your device too it. Accept the dialogue box on the device for USB debugging. Follow the instructions on the installer.
Before starting ensure the device is charged to over 80%. It uses a lot of power setting up.
Bugs on this device (the main ones) - battery drain, affects a lot of UT devices, it drains the battery quickly. If the battery is totally drained and you stick it on charge you will get a Qualcomm Crashdump notification. Leave it for about 20 minutes to 30 minutes then leaving it on charge, power up the device and it will reboot to Ubuntu Touch. When Powering off the device, Power off and leave the device for a few minutes. Then press the power button and Volume down button together until it reboots to the bootloader. Using the volume buttons, select power off - this will ensure the device is powered off. It doesn't quite power off properly so will run the battery down.
