Oppo owns OnePlus completely.

There used to be BBK Electronics that owned both, but it split up and OnePlus got placed under Oppo.

As other commenter said OnePlus is subsidiary of Oppo. Over past few years they were loosing autonomy / reusing more and more work by Oppo.

If you're worried about the firmware, then current day OxygenOS is just rebadged ColorOS. They just wont be pretending it's different now.

Only question/risk I see is Oppo trying to kill bootloader unlocking with an update.