Over in the OnePlus forums, one of the forum members has posted a link to an unofficial Android 6.0 Marshmallow update designed for the OnePlus One handset. This update is based on the CyanogenMod 12.1 Kernel, meaning that not only is this unofficially Google, but it is also unofficially CyanogenMod which we guess might put some users off.
However based on the feedback in the forums, it seems that the update is pretty well-received. Granted there are some bugs that might be annoying, but for the most part it seems to be relatively stable. Some of the bugs include how WiFi can sometimes cause reboots, NFC cannot be disabled, and no RIL.
If you’re fine with these bugs, or if you’re just curious and want to see what Android 6.0 feels like on your phone, head on over to the OnePlus forums for the details. Do take note that you will need a rooted device and an unlocked bootloader for this to work.