There is a new update rolling out for the OnePlus One, one that has been already made available for download by the company so that you can install it manually right away. While this doesn’t bring Android 5.1 to your OnePlus One handset, it fixes the critical Android vulnerability known as Stagefright.
The update is based on the company’s Oxygen OS, which means if you are still using Cyanogen OS, you will be switching to Oxygen OS upon installing the update.
You need to have TWRP recovery on your OnePlus One to be able to install the update. Get the TWRP recovery from here, unlock OnePlus One bootloader then, and then flash the TWRP recovery on it using fastboot.
Next, download the Stagefright fix update from here (685 MB), and transfer it to your device. Then reboot into recovery mode, and use TWRP’s install menu to install the update. That’s it.