Just a month ago, OnePlus was caught collecting personally identifiable data from phone owners through incredibly detailed analytics. While the company eventually reversed course on the data collection, another discovery has been made in the software of OnePlus phones. One developer found an application intended for factory testing, and through some investigation and reverse-engineering, was able to obtain root access using it.
The application in question is called 'EngineerMode,' which is intended to be used in factories to confirm that the device is working properly. We have confirmed it is installed on the OnePlus 3, 3T, and 5. It's even included on OxygenOS for the OnePlus One, but not the original CyanogenOS ROM.