OnePlus has revealed the official timelines for updating its smartphones to Android 6.0 Marshmallow. Both the OnePlus One and OnePlus 2 will be receiving the update in Q1 of 2016, but keep in mind that is a three month period. OnePlus One and OnePlus 2 owners can expect the following:

For the OnePlus One, Cyanogen OS will be updating to Marshmallow in the first quarter of 2016. We created a community build of OxygenOS for the OnePlus One, but this isn't what the product officially ships with. We will be updating the community build of OxygenOS for the OnePlus One when time allows.

The OnePlus 2 will also be updated in Q1, and the update will include the new standard Marshmallow API for the fingerprint sensor. We've heard requests for us creating an API for the current fingerprint implementation in OxygenOS, but have decided against this since we're switching over to the standard Android M implementation soon.

As for the recently-released OnePlus X, OnePlus has not yet committed to a timeline for the update. Be sure to stay tuned for additional information as it is available from OnePlus

Source: OnePlus