It doesn't make sense because OnePlus is much more known in the West than either Oppo, Vivo or Realme. OnePlus also just sounds like more of a Western brand.
It would have made much more sense to kill those other brands in the West and unify everything under the OnePlus banner.
To my ears, OnePlus sounds like a low-quality western brand. Along the lines of "Best Value" or "Farmer's Choice"
The branding logic actually makes sense from BBK's internal perspective — OnePlus, Oppo, Vivo, and Realme were originally separate fiefdoms under the BBK umbrella, each with its own P&L and channel strategy. When BBK restructured and put OnePlus under Oppo's management, the decision was driven by domestic Chinese market dynamics, not Western brand equity. Oppo's management likely saw maintaining a separate Western-facing brand as an operational cost that didn't justify the diminishing returns, especially once OnePlus lost its 'flagship killer' positioning and became just another rebranded Oppo. Chinese conglomerates often prioritise internal restructuring efficiency over international brand preservation — it's a recurring pattern you see across sectors, not just smartphones.