Android Marshmallow may be out in the world already but for right now it's only really available on Google own Nexus 5X and 6P handsets. Hopefully that will change as we've got an update on when the OnePlus handsets could be getting the new version of the operating system.
The OnePlus One will get its Cyanogen OS updated to a version incorporating Android Marshmallow in the first quarter of 2016, so by the end of March OnePlus One users will be right up to date. The plan is currently to roll out Marshmallow to the OnePlus 2 under the same timeline, with the phone also adopting thje new operating systems settings for the fingerprint scanner in the flagship handset.
OnePlus also just released a new smartphone in the form of the OnePlus X - which is smaller and cheaper than the rest of their range. There's no word on a release date for Marshmallow on the OnePlus X but expect more news in the new year as the company gets the rest of their new operating systems in order.