A big thanks to “bkchr” from the official forums, who posted the original guide: https://forum.kde.org/viewtopic.php?f=297&t=127502
The installation is done with Multirom, so you can install it beside your original Android rom.
First you need to install Multirom on your Oneplus One:
After installing Multirom, we need to install Ubuntu Touch, use the following guide
https://wiki.ubports.com/index.php/OnePlus_One (How to install Ubuntu Touch using MultiRom).
Select the latest stable version in the installer!
Start Ubuntu Touch for the first time and then reboot into recovery.
Install adb tools on your computer.
Now we need to get the system.img at your computer:
adb pull /data/media/0/multirom/roms/utouch_stable/data/system.img .
If you named your rom different, you need to replace “utouch_stable” with your rom name.
After the download finished(takes some time ~10 minutes), execute the following commands:
truncate -s +500M system.img
e2fsck -yf system.img
Copy the system.img back onto your device:
adb push system.img /data/media/0/multirom/roms/utouch_stable/data/system.img
Open a shell(adb shell) and navigate to the folder:
ls -il system.img ubunut.img
Check if both have the same inode(first number), if not
rm ubuntu.img && ln system.img ubuntu.img
Reboot to Ubuntu Touch and enable the developer mode, for the developer mode you need to have enabled a pin or password! (The pin/password is also you’re root password!)
Install “phablet-config” on your computer.
This will make your system.img writeable out of your Ubuntu Touch system!
After you’re image is writeable, open a shell at your device(adb shell), go to the following link and add the repos:
Replace “amd64″ with “armhf” in the first main repository!
Follow the script the script from line 39 until the end and do it by hand:
After that, restart the phone, you should hear a sound and plasma mobile should boot.