OnePlus has started rolling out Oxygen OS 2.2.2 update for the OnePlus X which adds new features, better camera UI but it's still based on Lollipop. Back in November 2015 the company had said that the OnePlus X will receive its Marshmallow update, but it seems that won't be coming anytime soon.
The 325MB update is available OTA and should be coming to most devices in the coming weeks. If you haven't got a notification yet, just head over to Settings > About phone > System update > Check for update.
Here's the official Changelog:
- Upgraded camera to version 1.4 UI
- Improved camera frame rate performance
- Added trusted face unlock security option
- Updated Swiftkey to version 6.3.3
- Updated carrier name mapping
- Added July security patches
- Fixed default SIM card selection issue
- Fixed network operator selection issue
- Fixed screen rotation issue
- General improvements and bug fixes
OnePlus has already said they are not going to make another OnePlus X and is just clearing up inventory at present.