Starting yesterday, OnePlus has started rolling out the second software update for the OnePlus 3T which brings Oxygen OS 3.5.4 on board. Unfortunately, the update is still based on Android 6.0.1 Marshmallow and brings minor improvements. OnePlus 3T has just landed in India and the device is priced at Rs 29,999 for 64GB variant and Rs 34,999 for 128GB variant. OnePlus recently confirmed that both the OnePlus 3 and the OnePlus 3T will get Android 7.0 Nougat update by end of this month so you will have to wait a little more to taste Nougat. 
Nevertheless, coming back to the update, OxygenOS 3.5.4 brings in optimizations for T-Mobile US (not valid for India), improves lag issue experienced when the battery reaches below 5%, improved Bluetooth connectivity for Mazda cards, WhatsApp flashlight usability fix and other bug fixes and improvements. Apart from this, the new update also brings an optimized battery saving mode for better battery life. Further, GPS and orientation settings will be restored once you exit the Battery Saving mode.
Meanwhile, OnePlus has also started rolling out OxygenOS beta 9 for the OnePlus 3. The is the second Android 7.0 Nougat beta built and brings in a bunch of features including support for expanded screenshots, battery optimizations, general UI improvements, improved DPI settings and other bug fixes. The original Android Nougat beta built was extremely stable and this new incremental update only adds to the stability. If you own a OnePlus 3 and are dying to use latest Android Nougat on your device, you will have to flash this version manually (no OTA) if you are running on previous non-Nougat beta versions. Also, once installed you cannot go back to Marshmallow without resetting the Operating System. For that, we would strongly recommend you to contact the OnePlus support to help you install the previous stable build again.
On the other hand, the OxygenOS 3.5.4 update for the OnePlus 3T will be rolled out in phases and the ETA (Estimated Time of Arrival) depends on the region you live in apart from company’s policy for incremental updates. At the time of writing this story, we didn’t receive the update but we expect to get it soon.