Earlier today a report come through detailing that it seemed likely that the Android 6.0 (Marshmallow) update would begin rolling out to the OnePlus One starting from tomorrow, April 9. At the time, the confirmation came by way of a blog posting which was released on the OnePlus India site. So it was unclear whether this was relevant to all OnePlus One owners worldwide or just India-located handsets. Since then, it seems more confirmations are starting to come through as the U.S. and Europe relevant OnePlus blog has now also confirmed the same details and that the update is rolling out from tomorrow.
In fact, the new message posted to the U.S./EU OnePlus site is essentially a republishing of the one which came through earlier. As a result there is no new information from this latest post, besides the reporting and confirming that the update is not only bound for Indian OnePlus One handsets. Interestingly, as this is essentially the exact same posting, it also comes with the same contradictory aspects, most notably, the referencing to "CM" instead of Cyanogen OS (or COS). Now, this is an aspect which is common to OnePlus and they have often referenced Cyanogen OS as CM, so in spite of the confusion with CyanogenMod, it is almost guaranteed that this update refers to Cyanogen OS 13 (Marshmallow) and not anything else. Although, the confusion has not been overlooked and has led to a number of replies on the most recent blog posting asking for proper clarification of whether it is Cyanogen OS or CyanogenMod.
Either way, it does seem pretty much confirmed that Marshmallow will be arriving tomorrow for the OnePlus One, in the form of Cyanogen OS 13 and build number ZNH0EAS26M. The blog posting does note that this is a staged roll out so it does mean that not everyone will get the update at the same time or even tomorrow. Instead, it will begin tomorrow and you can expect it to arrive at some point. Consistent with the earlier India confirmation, the current blog posting does not provide any further details on Marshmallow via OxygenOS for either the OnePlus 2 or the OnePlus X.