OnePlus has started rolling out the Oxygen OS 3.1.0 update for the OnePlus 2 based on Marshmallow.
The list of changes in the update include
Fixed some notification issues
Improved Doze Mode performance
Added media sound toggle for alert slider
Addressed alert slider/silent mode issue
Implemented August 1 security patches for Android
Implemented Quadrooter patch
General bug fixes
The update is now rolling out incrementally and should be available to everyone soon. Alternatively, you can download the update and flash it manually.