
OnePlus took to their own forums to announce that a community build of OxygenOS 3.1 (Marshmallow) is available to flash on OnePlus X handsets, indicating that a final build may not be too far off.
OnePlus has promised in the past the X would receive an upgrade to Marshmallow, but never provided a specific timeline for the firmware upgrade. Today, we're seeing the first community/test preview of Marshmallow for the handset. While this is most definitely a test build, it does indicate that a final build may not be too far out for OnePlus X owners.
Keep in mind that this truly isn't a final build, so you'll no doubt run into a plethora of bugs and possibly even some performance issues on the community build. It's also worth noting that those who flash the community build on their handsets will have to do a factory reset if they're coming from a custom recovery or custom ROM. You can grab the ROM from OnePlus here.

Be sure to act fast, as this is a limited run. Once OnePlus gets enough users running the beta, the download link will be disabled in order to prevent any additional users from downloading it.
Finally, there are some known issues you should be aware of:
- Mobile network stability issues in some regions
- Some 3rd party apps may experience some compatibility issues
- Language localization is still being improved
It's good to see that OnePlus is continuing to support the X, especially after announcing that the OnePlus X series wouldn't be continuing.
Now that we're seeing this community build, hopefully we'll see a final Marshmallow build for the OnePlus X by the end of the year.
source: OnePlus