OnePlus did promise company's OnePlus One handset will get Android 5.0 Lollipop as soon as possible and believe it or not, it is already here. I don't think anyone actually expected it to be available so soon, not even 24 hours passed since Google started rolling out OTA updates for the older Nexus devices. There is a catch though, even though this build appeared in OnePlus' official forums, this is not the official build. Not only that, but the uploader did say there are still a few bugs to fix, considering this is a first version of the ROM. Bluetooth doesn't work, for example. So if you use it on a regular basis (connect your Android Wear device for example), this ROM won't do you much good, you'd be better of waiting for Cyanogenmod to push the official build straight to your device. If you however don't need your bluetooth and are willing to try it out, here's how you can do that:
1. Download the Android 5.0 Lollipop ROM for OnePlus One here.
2. Copy the downloaded content to your SD card (do not unzip it).
3. Turn off your device and press and hold the volume down and the power button in order to boot into recovery mode.
4. Select clear data / factory reset (keep in mind that you should backup all your data beforehand, because flashing this ROM will delete everything).
5. Select "install from the SD Card", "update from the SD Card" or something similar.
6. Choose the ROM you downloaded.
7. Wait for the update to complete and then restart your phone.
8. You're all done.
I'll warn you once again, this is the unofficial build of Android 5.0 Lollipop for OnePlus One, so keep that in mind. Also, you're doing this at your own risk considering the fact you're manually flashing an unofficial ROM on your phone, so if you're not comfortable doing that and / or haven't done it before, I really don't recommend it, you're better of waiting for the official version of the ROM. That being said, I haven't flashed this version on my own considering I don't own the device so I can't exactly share my experiences, but it seems a ton of other people has and according to them the ROM is working just fine.