As promised, OnePlus has added VoLTE support on the OnePlus 2 with a new OTA update which is currently rolling out which would be greatly appreciated by Jio users in India. The Oxygen OS 3.5.5 update, also brings the December security patch from Google but is still based on Android Marshmallow.
Some new features that have been added include App Lock, so that you can secure apps using the fingerprint sensor and there are a few Alert Slider options as well. The Shelf has gotten a makeover too. There's a Battery Saving mode as well as a Gaming Mode making it to the new build. One thing you should note before installing the update is that the stock Clock, Calculator, and Messages app will be replaced by OnePlus apps if you have not updated these. But you can always download them again from the Play Store.
Recently OnePlus also updated the OnePlus 3 and OnePlus 3T with beta Nougat builds, and here's a handy tutorial to install Nougat on your phone. Check Out: How to Install Android Nougat on OnePlus 3 or OnePlus 3T