Cyanogen is rolling out its OS 12.1.1 update to One plus one. In this update Cyanogen OS with support for Microsoft Cortana. Miocrsoft Officially made Cortana for other phones the OnePlus One is unique one to get it first.
Now, as with Google Now, users will be able to say “Hey Cortana” and make use of the voice search features for their web searches as well as set alarms, set quiet mode, and change settings such as data or airplane mode, thanks to the deep integration of the feature into the OS.
If your do not want to use Cortana you can disable it on your device and use Google Now. But you can only have one or the other enabled at time. The update is available in the system menu, but be aware the “12” in the version indicates this is still Lollipop, not Marshmallow. Moreover Cortana will be limited to the US for now.