OnePlus seems to be moving pretty fast with the updates for its latest handsets, although it might take a while until the Android 7.1.1 Nougat-based firmware reaches its final form. For now, both OnePlus 3 and OnePlus 3T receive a new beta firmware build.
For now, these updates are known as Open Beta 3 (OnePlus 3T) and Open Beta 12 (OnePlus 3). However, their highlights are the same:
- Android 7.1.1 Nougat
- New Shot on OnePlus wallpaper feature
- multiple Gallery improvements
- general bug fixes
The new firmware is about 1.5 GB in size and, sadly, it also comes with a few known issues. Fortunately, there are only two problems, and one of them already has a fix available - Android Pay not working on London TFL/Tube can be easily solved by updating the app via Google Play, while the compatibility issues encountered with certain third party apps should be ironed out until the firmware gets out of beta.
Source(s)
OnePlus Forums (OxygenOS Open Beta 12 for OnePlus 3)
OnePlus Forums (OxygenOS Open Beta 3 for OnePlus 3T)