OnePlus One owners have had to wait a long time for their Android 5.x (Lollipop) update to arrive. That said, it did and did so in quite dramatic fashion with two competing OSes arriving pretty much at the same time (give or take a week). These came in the form of the Cyanogen based offering, Cyanogen OS 12 and their own homegrown offering, OxygenOS. In terms of the first, Cyanogen OS 12, was the follow-up to the operating system that comes pre-installed on the OnePlus One and therefore, users were eager to see what the beloved operating system had to offer their devices in an upgraded form. In terms of the latter, there was just as much interest in OxygenOS, due to this being the debut operating system from OnePlus.
That said, they are not the only two operating systems available for the One. Beyond the numerous ROMs and OSes available, OnePlus also released another version of their homegrown OS in the form of Hydrogen OS (H2OS for short). As was covered quite extensively when Hydrogen OS was first unveiled, this is meant more for the OnePlus domestic China market, more so than the international one. However, for those that found OxygenOS too lightweight and too close-to-stock, then Hydrogen OS might prove to be the perfect alternative.
Well, if you have been waiting for a good opportunity to try out Hydrogen OS, then now might be the time. Although, Hydrogen is still in Beta mode (expected release next month), version 4 of the Beta became available last week for download. One of the folks over at XDA has made the link available and therefore anyone wanting to try Hydrogen OS in its latest form, now can. Lastly, a word of warning. As well as being a beta form, the update seems to be problematic for some users and therefore you would be wise to read the information on XDA (source link below) prior to flashing the OS. Not to mention, make sure you are protected from all nasties including bricking (which some are claiming has happened). As always with these things, you install at your own (and your phone's) risk.
Hydrogen OS V4 - Via XDA