Even though it was delayed, the folks at OnePlus wanted to reassure their customers that the OnePlus One and the OnePlus Two will both receive their Android 6.x Marshmallow update sometime in the first quarter of 2016. Here's a quote from their official statement,
For more info and discussions on both of these devices, here's our dedicated section: OnePlus Smartphones | Android Forum at DroidForums.netFor 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. ~ OnePlus