The OnePlus X is released the latest update of Oxygen OS 2.1.3 (OTA). OnePlus X is actually making some improvements on fixing SD card issues(extFAT), found it one of the forum of OnePlus X forum.
The OnePlus X was having some issues with SD card. rebooting issues when removing SD card when the phone is active, moving apps to SD card from phone storage. And they going to fix it - "We've heard your feedback and spent some time focusing n improving the user experience for SD card[s]. We are not done yet so please expect more updates and feature additions to come in the next couple of OTAs..." posted by the OnePlus forum.
The update will increase the stability of the phone and also fix some bug issues, which was there in the older version of Oxygen 2.1.2 update.
OnePlus X is released the update Oxygen OS air(OTA) on 27th of November. And soon all will get the update. And they also said that this is just a beginning of updates.
OnePlus is also talking about the Marsmallow Update in the 1st quarter of 2016. The company yet have not decided particular date or month. But as this post of the company tells more about it. The OnePlus posted in forum that - "Many of you have been wondering about Android M and when exactly it’s coming to your OnePlus device. Although we don’t want to promise specific dates yet, we want to share some more detailed answers with you today."