As recent history lets us know, the original OnePlus One was initially powered by CyanogenMod 11S in collaboration with Cyanogen Inc. but eventually OnePlus decided to switch gears and develop its own in-house mobile operating system for future models. In fact, OnePlus has developed two versions of Android for different markets, and while most OnePlus owners around the world have experienced their devices running OxygenOS, customers in China have been treated with HydrogenOS: a custom version of Android with a heavily modified user interface. Now, earlier this month OnePlus confirmed that its two ongoing Android platforms will merge in order to bring OS update release timeframes closer, and since then many OnePlus customers have been wondering if the company will maintain a user interface closer to stock Android, or adopt the ways of HydrogenOS entirely.
The answer to these questions was provided by OnePlus in a recent Reddit AMA (Ask-Me-Anything) which clarified the company's plans for its custom operating systems. Specifically, the merger of OxygenOS and HydrogenOS will not result in a single operating system across all OnePlus devices, but instead, the company will keep both versions separate, at least as far as appearances are concerned. This means that although the company will continue to merge the backend of the two operating systems, the overall look and feel of OxygenOS and HydrogenOS will remain different. According to OnePlus "We['ve] combined platform, system and framework already. Application layer is separated. So, global users will still have the Oxygen experience and users in China will have Hydrogen experience."
In other words, the goal of merging the backend of the two operating systems is to allow the company to bring software updates to both platforms sooner and roughly at the same time, and continue to support the two OS versions on a plain field. However, it appears that these changes will not affect the design philosophies behind the user interface of each OS version, so we can expect OxygenOS to continue and offer a near-stock Android experience sprinkled with useful custom additions and features from OnePlus. On the other hand, HydrogenOS will probably remain a more heavily-modified version of Android, at least on the face of it, and will continue to appeal to the Chinese market.