OnePlus X is finally getting the Android Marshmallow upgrade. Its producer has confirmed the reduce of OxygenOS version 3.1.2, the first official Marshmallow build for OnePlus X. To update OnePlus X to Android Marshmallow, it should be running on the latest OxygenOS 2.2.3. OnePlus recently rolled out the 2.2.3 update for the device, which includes fundamentals for Android Marshmallow update. If you have not updated to OxygenOS 2.2.3, just update your OnePlus X to the version (guide here) and then check for the OxygenOS 3.1.2 (Marshmallow)OTA update by going to Settings > About Phone.
How to install OxygenOS 3.1.2 on OnePlus X (for non-rooted)
If your OnePlus X is non-rooted and running on stock recovery, then all you need to do is download the update, reboot phone into stock recovery. This will automatically install the update.
How to install OxygenOS 3.1.2 on OnePlus X (for rooted)
- Download OxygenOS 3.1.2
- Put the phone to custom recovery (TWRP)
- In recovery mode, perform a full data wipe such as wipe system, data, cache, and dalvik cache
- Select Install then browse to the location where you have saved the downloaded file, select it and confirm installation
- In the end reboot the phone