One of the main reasons that OnePlus gained popularity early on is the fact that they have always been so developer friendly. For this reason rom developers gravitate toward their phones. This also means that rom development is pretty rapid and continues to happen as the devices age. You just really don't see the type of developer community engagement with other flagship phones as you do with the OnePlus series.
The OnePlus 3T now has an unofficial build of CyanogenMod 13. The rom supports both the OnePlus 3 and OnePlus 3T. The CM13 builds are based on Android 6.0 Marshmallow, and feature a custom kernel as well. It should also be noted that these unofficial builds are based on the stable branch of CM and not the nightly builds which means the rom should be much more stable. Of course you get all the extra features from CM and some custom features from the kernel that improve on stability and even battery life. Head to the link below for downloads.
via XDA