Cyanogen, Inc has been partnered with Microsoft for a while now, bringing various mods to devices running Cyanogen OS. The company is launching the Cortana mod in India today, starting out with the OnePlus One. Cortana first came to the Cyanogen operating system in January this year, followed by the 13.1 update, which enabled more widespread mod support.
To get this update, your OnePlus One must be running the latest version of the operating system - the aforementioned 13.1. This enables the voice-activated assistant feature which behaves much like Google Now; Cyanogen says Cortana is deeply integrated into the OS, with a 'Hey Cortana' command waking her up. As with Now, saying things like 'Hey Cortana, set an alarm' or 'Hey Cortana, turn my WiFi on' will do those things without need of the touchscreen at all. There's also the camera mod, with a 'Take a selfie' ability which counts down to taking a hands-free picture, and a lockscreen mod which has voice search, again rivalling Google Now.
The update is available in India right now - Cyanogen says the phone needs to be connected to a WiFi network and will download automatically.