OnePlus One smartphone users can now upgrade their device to Android Lollipop with official Android 5.0 Lollipop ROM. OnePlus has developed its own Android 5.0 Lollipop ROM for the OnePlus One phone, may be due to the controversy with CyanogenMod licensing in India. Based on the ASOP (Android Open Source Project), this ROM is free from bloatware apps and has near stock interface, like Nexus devices.
The following guide shows you how to install official Android 5.0 Lollipop ROM on OnePlus One. However, please note that this is an experimental build. It might have many bugs that have not been fixed yet. According to OnePlus, the major bugs on this ROM are unstable camera, video and Wi-Fi functionalities and have slower startup time on first boot; however, the developer will fix all the issues soon.
Before you get start, keep in mind that you will not be able to receive any over-the-Air (OTA) updates after installing this ROM on your phone. Also, your device’s warranty will void, so install at your own risk.
Important: As always, backup entire data on your phone before installing this ROM. Also, make a full system backup (Nandroid backup) from custom recovery.
1. First, install TWRP custom recovery on your phone [Download here]
3. Turn off your phone and boot it into TWRP recovery mode. To do so, press and hold Volume down and Power buttons together.
4. From recovery, do a factory reset, cache, and Dalvik cache.
5. Now, go to Install and locate the ROM zip file you copied to your device and confirm flash.
6. If you have GApps (Google Apps), install it too. After flash is complete, clear your cache again. This will fix boot loop issue. Then, reboot your phone.
That is it. It will take little more time for the first boot, so be patient. Enjoy Lollipop on your OnePlus One phone (source)