OnePlus is very busy at the moment as the company moves to get the OnePlus 2 into the hands of millions who have ordered the device. OnePlus is trying to avoid problems it faced with the OnePlus X distribution, which is probably why the OnePlus One is still stuck on Android Lollipop.
From what we have come to understand, there's no official Marshmallow build available for the OnePlus One, but do not fret because the OmniROM team might have what you're looking for in the form of Android 6.0.
This is not official so folks who are willing to jump in should expect a few bugs and other problems. Still, the development team at OmniROM is one of the best out there right now, as there are several custom ROMs for various devices under their belts.
Some folks may be worried about potentially voiding the warranty of their OnePlus One should they install an official Android 6.0 Marshmallow ROM. Well, the terms and conditions with the OnePlus One allow the unlocking of the bootloader and adding of custom ROMs, but rooting does typically void warranties.
Bear in mind that OmniROM requires a custom recovery and it also requires TWRP.
The OmniROM can be downloaded right here from the XDA-Developers website. Instructions and everything else you'll need to know is right there.
The OnePlus One was released back in 2014, and it is still one of the best Android smartphones available right now. The handset comes with a 5.5-inch touchscreen display, Corning Gorilla Glass 3, and a Qualcomm Snapdragon 801 processor.
Furthermore, there a 13-megapixel camera at the rear, and it is capable of taking 2160p at 30fps, 2160p at 24fps, 1080p at 60fps, and 720p at 120fps videos. On the front, there's 5-megapixel camera for taking selfies and making video calls.
As for the memory department, the OnePlus One is rocking 3GB of RAM and 16GB of internal storage with the option to increase it with a microSD card.