Cyanogen Inc is rolling out Cyanogen OS 12.1.1 for the OnePlus One today, and it's not just any update. This is the first version of Cyanogen OS with support for Microsoft Cortana baked right in. After Microsoft finally made Cortana official for other phones, then removed the always listening feature, the OnePlus One is rather unique.
Cortana on the OnePlus One will have the "Hey Cortana" hotword as well as support for alarms, quiet mode, and settings toggles by voice. Cyanogen Inc. says the deep integration with the OS will allow Cortana to do more things in the future too.
The update is available in the system menu, but be aware the "12" in the version indicates this is still Lollipop, not Marshmallow. Additionally, Cortana will be limited to the US for now. If you don't want Cortana on your device, it can be uninstalled after the update.