OnePlus revealed that the company will be merging its global Oxygen OS and Chinese Hydrogen OS, in an interview with xda-developers.
Hydrogen OS is the company’s ROM exclusively for the Chinese market, featuring a skinned UI more in tune with the tastes in the Chinese market, whereas the Oxygen OS for the global market has a much more stock UI. Going forward, the company will be merging the two teams as well as the two products. Having both the teams working on a single product should produce better results and faster turnaround time for updates. This also means that the end product should have the best (hopefully) of both worlds, as now there will just be one ROM for every market.
OnePlus recently released OnePlus 3 OxygenOS 3.5.1 community build, which gives users a first look at what to expect in future. This is the first product that is the result of the merger of the two teams. It is still very much a work in progress, though, and future releases should include more changes, for better or worse.