Hydrogen OS 3.0 for OnePlus 3 and 3T is now rolling out. Hydrogen OS 3.0 is based on Android 7.0 Nougat and is primarily intended for Chinese customers.
The new update brings a redesigned UI, call recording functionality, bug fixes, performance enhancements and battery life improvements.
That said, it is substantially different from its sibling branch, Oxygen OS, which is for international customers. Although Hydrogen OS generally contains more fixes and improvements, it is important to note that being a Chinese build, it comes devoid of any Google services, including the Play Store.
However, it can be flashed on any OnePlus 3 or 3T device. The ROM for Hydrogen OS 3.0 weighs in at 1.2GB in download size and rolling out over OTA. You will find more details and download link here.