OnePlus has today official announced plans on when the OnePlus One and the OnePlus 2 will get the Android Marshmallow update. According to OnePlus the update will first hit the OnePlus One via Cyanogen OS and that will begin rolling out in Q1 of 2016. This means the update will hit between January 1st and March 31st. An update to Oxygen OS will come after the update to Cyanogen OS.
For the OnePlus 2 we can expect the update to hit in the same time frame, but in this case directly through Oxygen OS. The OnePlus X will also be updated to Marshmallow although a time frame for that has yet to be released. While users are waiting OnePlus will continue to roll out maintenance upgrades for Oxygen OS as usual. The fingerprint API OnePlus had been working on has also been scrapped in exchange for the official API now built into Marshmallow.Source: OnePlus