There are not many of these models, but for their holders the information is rather important. All OnePlus smartphones will get an update to Android 6.0 Marshmallow, as the manufacturer announced today on its forum.
Cyanogen OS in OnePlus One will be updated to Marshmallow in the first quarter of 2016. What is true for OnePlus One there was created separately build OxygenOS, but this is not the final version for sale. The manufacturer promises that OxygenOS for OnePlus One will be updated as soon as time permits.
OnePlus 2 will get the upgrade to Marshmallow also in the first quarter and the update will include a new API to use a fingerprint reader. Here, the manufacturer also informs that it knows the wishes of the community for the creation of API for the current OxygenOS version, but it will not decide on such a move, which is due to a rapid change to Android M.
Regarding OnePlus X there was not even mentioned the approximate date of update.
Source OnePlus