With the latest update to OxygenOS 2.2.0, the OnePlus X now properly displays its Android security patch level in the settings separate from the rest of the software information. The phone is currently on the January 1, 2016 release, which means at least for the next few days (assuming Google sticks to the monthly schedule) it's on the latest security patch.
Though the phone is still running on a base of Android 5.1.1, like many other manufacturers OnePlus has chosen to apply these monthly security patches on its pre-Marshmallow phones. We'll have to see how well OnePlus can keep up with the monthly cadence of the updates, or if it will start to fall behind as some other phone makers have.
The OTA to the latest update is currently rolling out to all OnePlus X phones.