If you plug your OnePlus Two or OnePlus X on a Ubuntu 14.04 LTS workstation while in MTP mode, you'll notice that you won't be able to access your phone filesystem. You'll only get a read-only cd-rom emulation drive holding some windows and OSX drivers. But if you connect in PTP mode, no problem, everything works fine.
This comes from the fact that OnePlus Two and OnePlus X are not known as a MTP device under Ubuntu 14.04 LTS.
You'll also realize that your OnePlus Two / OnpePlus X won't be recognized as an ADB device, even if ADB has been enabled on the phone. Thats again because vendor ID is not included in the default list.
This short article explains how to declare your OnePlus Two / OnePlus X as an ADB and a MTP device on your Ubuntu 14.04 LTS workstation to get a fully functional access to your device.