OnePlus has two operating systems for its phones: there is Oxygen OS for the international variants, and for the domestic variant you have Hydrogen OS. While owners of the OnePlus 3 and OnePlus 3T running Oxygen OS have already received the Android Nougat update (Oxygen OS 4.0.3) for close to two months now, those on Hydrogen OS are just getting the Nougat treatment in form of Hydrogen OS 3.0.
Although, Hydrogen OS is built for users in China, it is also available in English. Like most Chinese forks of Android, you can sideload Google Services after flashing it on your phone and unwanted Chinese apps can be uninstalled. Some users say Hydrogen OS has a better battery life than Oxygen OS and there is also call recording. There is even a poll on the OnePlus forum to determine the better one.
The update file is 1.2GB large and can be downloaded on the OnePlus site or on XDA. There are installers included to help you sideload Play Store in case you want to give it a shot.