Google has already given us the developer preview of Android N, yet only a small percentage of devices have actually made it to Android 6.0 Marshmallow. The Nexus lineup and a few select devices from OEMs like Samsung, LG, HTC, and Motorola have received the update. You can now ad OnePlus's One to the list.
The CM13 OTA rollout for the device began yesterday and will update users from Lollipop to Marshmallow. Of course if you aren't willing to wait on the OTA release you can manually flash the rom for yourself.
via OnePlus India Blog