Before the starting of 2017, OnePlus started rolling out OxygenOS 4.0 based on Android 7.0 Nougat update for the OnePlus 3/3T. But, now there's a new Nougat firmware for the OnePlus 3/3T, and it's not Oxygen.
The company has released Hydrogen OS version 3.0 which is based on Android Nougat for the OnePlus 3 and 3T. This custom rom is for their Chinese customers with Chinese and English language support. The rom has no Google apps included, but you can download and flash it manually.
Download Hydrogen OS 3.0 for the OnePlus 3/3T here-
Hydrogen OS 3.0 for OnePlus 3: Download Here
Hydrogen OS 3.0 for OnePlus 3T: Download Here