You understand that OnePlus is right now busy trying to get the OnePlus 2 into the hands of the millions who ordered and bought them, while also trying to make sure they don’t make the same shipping and manufacturing mistakes they made for the recently-launched OnePlus X. With an outfit that small, you can understand why the OnePlus One is still on Lollipop, and no official Marshmallow build out yet.
Well, that’s when you turn to the aftermarket development community, and the OmniROM team may just have what you need – an Android 6.0 build for the OnePlus One, give or take some bugs. The OmniROM team is one of the more decent development teams out there, making custom ROMs for different devices – this one a Marshmallow build for the OnePlus One.
As always, you void any sort of warranty when installing a custom ROM, because it needs a custom recovery – and OmniROM requires TWRP. This may require you gaining root access, so in any case, your warranty is void. There may still be some bugs to this, so you have to report it to the developers via the official XDA thread (see source link below). Download links for the ROM are also there.
The OnePlus One is actually one of those flagship phones that work well on Lollipop, so you might just choose to wait it out until the official update arrives. Then again, you might be tired of waiting. So whatever you choose, make sure you do it right and clean.