Since the update is coming straight from OnePlus, it is being sent out OTA. Just the other day, a community sourced version of OxygenOS 2.1.4 was made available to OnePlus One users. Since that update isn't coming directly from the manufacturer, it has to be flashed to the OnePlus One.
If you own the OnePlus X and don't receive the update right away, you need to be patient. OnePlus is sending it out in waves so that if a problem pops up, it can be more quickly dealt with. When you are notified that OxygenOS 2.2.0 has reached your handset, make sure that you are using a Wi-Fi network, and that your battery is charged to at least 50% before starting the updating process.
The OnePlus X is the third phone from the manufacturer, following the OnePlus One and the OnePlus 2. Created for those favoring form over function, it is powered by a Snapdragon 801 chipset (no, we didn't transpose the digits) and carries a 5-inch screen with a 1080 x 1920 resolution. The handset was launched last November.
source: OnePlus via MobileSyrup