OnePlus just confirmed that they will provide us with Lollipop for One in March. There would be two versions - Oxygen OS and CM12S. OxygenOS is the work of Paranoid Android people and it's very promising. We will get it first, on March 27, but it's won't that easy to update your smartphone with that ROM. I will make video tutorial once the Oxygen OS is available.
CM12S will came on March 30 as an OTA update, so you will get it automatically. Not everyone will get it at first day, but I will provide you with link to OTA and tutorial how to update your smartphone. Stay tuned for more info!
Official OnePlus announcement:
We know there is a lot of excitement in the community surrounding the Lollipop updates for the One, and for good reason. We are just as excited as you are.
We never intended to keep you waiting. Unfortunately, we ran into some unexpected roadblocks. First, there was an unforeseen OS signing issue, which has recently been resolved. Our camera module had stability issues on Lollipop, and we had to rebuild it for compatibility. Several other crucial drivers, including our Wi-Fi drivers, also needed to be updated to work flawlessly with the upgrade. Admittedly, it took a little more time than we had planned, but we only release updates that we are proud of; that means refusing to take shortcuts or deliver sub-par experiences.
With all of that in mind, here are the official release dates for Android Lollipop on the OnePlus One:
CM 12S: The official OTA update for the OnePlus One will be released on or before March 30, 2015
OxygenOS: The official release of our own Lollipop-based ROM will be available for download on or before March 27, 2015
Source - OnePlus.net