Even though Cyanogen OS 13 has already been available for OnePlus One a few weeks ago, Cyanogen Inc. has not announced it officially until now. Now the company officially announced Cyanogen OS 13 and the roll out is imminent for supported devices. Definitely, the first device to get Cyanogen OS 13 (Android 6.0 Marshmallow) will be OnePlus One, followed by other Cyanogen OS powered devices, such as ZUK Z1, Yu Yureka, Yu Yuphoria, etc.
Based on the Android 6.0 Marshmallow, Cyanogen OS 13 will bring various new features, performance improvements and latest Android security patches to supported devices. Doze battery saving, Permissions control, Privacy Guard, App Standby, Quick Settings tiles, Truecaller mod for Messaging and plenty of other new and improved features will be included in this OS update.
For your information, Cyanogen OS is a commercial version of CyanogenMod, that comes pre-installed on some devices such as OnePlus One, ZUK Z1, Yu Yureka, Yu Yuphoria, etc. The two are broadly the same in terms of features, but CyanogenMod offers faster and more regular updates.
As said above, Cyanogen OS 13 OTA (over the air) update has already been available for OnePlus One [Read this guide to know where to download and update manually] and the latest version update your phone to Cyanogen OS 13 version ZNH0EAS2JK. Read this guide to know how to update to latest Cyanogen OS 13.
Note that, currently only OTA (over the air) update for Cyanogen OS 13 is available, so your OnePlus One should be running on the latest Cyanogen OS version (Cyanogen OS 12.1 YOG7DAS2K1) to apply the update.