OnePlus One was the first device by OnePlus and it ran on Cyanogen OS. The deal between Cyanogen OS and OnePlus excited many. However, they had a bitter fall out and after OnePlus One, Cyanogen OS was not seen in future OnePlus devices. But Cyanogen OS has continued support for OnePlus One. And now they are releasing OTA for OnePlus One in the form of Cyanogen OS 13 i.e. Android Marshmallow.