NMIXX is, unsurprisingly, continuing the current kpop trend towards chaos-pop recently popularized by Aespa: pop music designed to continually surprise the listener by switching up the sound, at the cost of coherency.
All previous kpop was basically a mix of different styles started outside of Korea, but this new style of pop seems uniquely Korean (mixing sounds repeatedly and dramatically within a single song - not just at the drop). Electronic music has done stuff similar to this for a long time, but I've never heard anything quite like this in pop.
I Got a Boy (SM vocal drop), Come Back Home (Teddy drop) and then Bang Bang Bang (Teddy drop, and a big switch at the end) did something kinda similar , but even that was much less extreme than this new stuff. It's like SM and now JYP are taking the direction that SM and Teddy was going in the last decade and going even further with it.
All genres start as a mix of existing genres. The sound mutates as more artists experiment with it, eventually becoming different enough from the influences that it becomes a new genre.
Basically, I'm saying that O.O is a mutant that seems to be leading to a new genre, but it isn't quite there yet.
So the thread doesn't get derailed, my point isn't about who started moving in this direction. The point is that if they keep going further in this direction it is arguably a new, and uniquely kpop, genre.