Below are dates of official Android marshmallow release for Oneplus devices.According to latest announcement by oneplus team.
According to oneplus team the first Device which is going to Get Android marshmallow 6.0 Update is OnePlus one it will still on cyanogenmod Os.So The Update will be of Cm13.The date of Android marshmallow release for Oneplus One is in the first quarter of 2016.
The OnePlus 2 will also be updated in Q1, and the update will include
the new standard Marshmallow API for the fingerprint sensor.
Where as Oneplus X marshmallow update date is not yet fixed by oneplus team.They are working hard to bring android marshmallow 6.0 oxygen OS to Oneplus X soon.So keep visiting for an update on it.Kernel sources for the OnePlus X Is also released here: https://github.com/OnePlusOSS/android_kernel_oneplus_msm8974