Just a month ago, OnePlus released "Hydrogen OS" for the OnePlus 3/3T. Hydrogen OS is the custom rom for Chinese customers with Chinese and English language support. The rom has no Google apps included, but you can download and flash it manually.
Now, the company has started rolling out Android 7.1.1 Nougat with March 2017 security patches via Hydrogen OS beta. The firmware is now live and is available at H2OS website, it costs 1.32GB.
We have no information on when OnePlus will start testing OxygenOS based on Android 7.1.1 Nougat. Well, for now OnePlus users can taste HydrogenOS with Android 7.1.1 Goodies!
Download Hydrogen OS (Android 7.1.1) Beta for the OnePlus 3/3T here-
Hydrogen OS 3.0 for OnePlus 3/3T- Download Here