CyanogenMod team released the CM13 source code over the weekend, and since then there have been many unofficial ports of CM13 to various Android devices over at xda and other Android communities.
For the OnePlus One users, Cyanogen isn’t alien, the device itself runs on Cyanogen OS. However, CyanogenMod and Cyanogen OS are developed differently. If you hoped the release of source code for CM13 means you’re soon going to get the Marshmallow update to your OnePlus One officially, then you’d be wrong. But unofficially, yes.
You can update your OnePlus One to Android 6.0 Marshmallow via CM13 or an AOSP ROM. CM13 is yet to officially make it to Android devices, which will probably start with Nexus phones, but OnePlus One will be one of the first after Nexuses get it.
You can grab OnePlus One CM13 ROM and Marshmallow Gapps from the download links below. And follow the link to the guide to safely install CM13.