There was a bit of a scandal when they started merging their Chinese OS with their American OS. I think they were called Hydrogen and Oxygen. I haven't used a OnePlus phone since then so I'm not totally sure what their software is like nowadays.
I'd rather not support a Chinese company, but since no one else will sell a phone with descent specs with a microsd card, I'm going to be forced into it sooner or later. My n20 ultra is still doing fantastic for now, though.