Most flagship phones from last year have gotten their Marshmallow updates by this point, but not the OnePlus 2. Technically, it's not a flagship—it's a flagship killer. OnePlus took time out from killing flagships in March to release a community build (beta) of Marshmallow for the OP2, and now there's an update to that in the form of OxygenOS 3.0.1. Guess what, it's still just a community build.
Here's what you can expect to change in this version of OxygenOS.
Enhanced Doze mode to better optimize battery usage
Improvements to the fingerprint sensor for faster and more accurate recognition
Added clear process in the recents screen for the option to remove background processes
Added feature in recents screen to allow you to lock apps that you don’t want removed with clear all
Bug fix in network operator selection to ensure the selection sticks to your choice
Swiftkey update to version 6.3.3
Added back display color balance
Updated carrier name mapping
Camera improvements including bug fixes for OIS
The new build is only available as a full ROM, which has to be flashed from official Oxygen recovery. You must be running OxygenOS 2.2 or higher as well. If it's anything like the last build, you should get an OTA update to the final version whenever that comes out. I would say it shouldn't be much longer, but you would have thought that back in March too. Frankly, it's embarrassing that OnePlus still hasn't released the OTA at this point.