I worked at OPPO and OnePlus when OnePlus was being re-integrated into OPPO. It used to be its own org with a separate office, but the team was moved into the new OPPO office.

If you want to understand why OnePlus has been destroyed since then, there's a number of factos that I will just quickly list: OPPO was doing a large international push from 2019, and OnePlus was the little sub-brand that was more successful than it's mother brand. OnePlus was an international team, OPPO fully Chinese, with a CEO and the entire upper management not able to speak English. OnePlus continued to succeed while OPPO couldn't do good marketing. This was embarrassing, so of course OnePlus had to be downgraded. Pete Lau became OPPO CPO and decided that OnePlus should be a lower to mid brand, and the product was changed to reflect that. Lots of people left.

Ego, incompetent management that knew nothing about the world outside of China and terrible decisions that followed, led to the erosion of OnePlus.

It could have made them a real player internationally but they intentionally destroyed it.