So its been a while since I updated my phone firmware, for some reason on the OnePlusOne if you have a custom firmware, you still get prompted for an update, but it fails to install (most likely because of the custom firmware.
I decided to give OxygenOS a go, which is OnePlusOnes own take on CyanogenMod. Here's how I installed the OS, in case you didn't know already...
The build I used was the official v1 of OxygenOS, which can be downloaded here:
You have to make sure all the data is backed up before starting, as the flashing process will wipe the data from the drive. The image needs to be flashed manually, there isn't an automated installer.
This tutorial is written for OxygenOS the OnePlus One device, so it shouldn't be tried on any other device, or you could end up with a brick. The good thing is, flashing will not void your OnePlus warranty, but I've read that any damage caused by improper flashing is not covered under warranty.
Next is installing the actual OS.
Unzip the zip file you just downloaded. Inside the zip, you will see a file called oxygenos_1.0.0.flashable.zip. Copy this file to the root of your phone.
Next boot into recovery mode. This can be done by holding down both the volume down key and the power button for a few seconds.
Next you need to do a factory reset, which will lose all your data, but not contents of the drive. This can be selected on the TWRP interface
Next install the file you copied over to your phone and click confirm flash. This will flash the operation system onto your device. Next reboot the device,
When the phone next boots up, you should have OxygenOS!