It seems that OnePlus is working on Android Marshmallow based OxygenOS 3.1 for OnePlus X. A community build of OxygenOS 3.1 for OnePlus X is now available for download, which brings Android 6.0.1 Marshmallow and various new features to the device. Interested users can tryout this OTA eligible OxygenOS 3.1 community builds right now.
Last month, OnePlus 2 has received a community build of OxygenOS 3.0 (Android 6.0.1 Marshmallow) for testing purpose and now it is updated to OxygenOS 3.0.1 with new features and bug fixes. As the OS is OTA eligible, users will receive the OTA (over the air) update for this build. The OTA roll out will start in coming days, so wait until you are notified.
OnePlus team is working on OxygenOS 3.0 (Android 6.0.1) for OnePlus 2 and other OnePlus phones and the soak testing already started a few weeks ago. Now, a community build of OxygenOS 3.0 is available for OnePlus 2, which you can test out now, if you cannot wait for the official roll out. OnePlus is releasing this version, as they want feedback from users on bugs.