If you've ever had the pleasure of using a custom Android ROM then you'll know this statement to be true: ROMs made by Android OEMs largely suck.
OnePlus avoided this fate by shipping their "flagship killer" with one of the slickest third-party ROMs out there (CyanogenMod), tweaked even further (Cyanogen OS) to support 4K video and various gestures to wake the device from sleep. The OnePlus One, already an attractive device, was made pretty much irresistible with Cyanogen running it.
And then, the relationship between Cyanogen, Inc. and OnePlus went very quickly and publicly
Cyanogen has promised to release at least one Lollipop-based ROM specifically for the OnePlus One; in the meantime, OnePlus has released their own 5.0 ROM called OxygenOS—in other words, another OEM Android ROM.