All it takes to turn an OPPO phone into a OnePlus device is...a letter to the FCC?
The letter notes that OnePlus wants to rebrand the device as its own.

OPPO has informed the FCC that it has allowed OPPO to market CPH2387 as its own device
When OnePlus started out, it focused on the enthusiast market and had an identity of its own. This began to change by the end of 2020 and OPPO and OnePlus later merged their R&D units. OnePlus' software overlay OxygenOS began to reek of OPPO's ColorOS, but the backlash that the companies received for this was so severe that it was announced that though the two will continue to use the same codebase, the two operating systems won't be unified.The two will still likely continue selling each other's products with a few tweaks here and there. For instance, one rumor says that the OPPO Find N will be a blueprint for OnePlus' first foldable phone. Another leak claims that the OPPO Reno 8 could be a OnePlus 10 Pro knockoff. The FCC document mentions the changes that OnePlus will make to the OPPO CPH2387 to transform it into a OnePlus device. The first, and most important thing is that the model number will be changed to CPH2469 and the company name will be changed to OnePlus.The battery cover will be stamped with a OnePlus logo and the color of the USB cable will be changed to red from green. The main camera will be replaced with a higher resolution 50MP sensor and the device will run OxygenOS instead of ColorOS. The dimensions remain the same at 163.74 x 75.03 x 7.99mm and the battery capacity also remains unchanged at 5,000mAh.

The mystery OnePlus Nord device will essentially be a Oppo A57 4G with a different main camera
Not much else of importance is mentioned in the documents but if you are wondering what the mystery OnePlus phone could offer, the Oppo A57 4G features a 6.5-inches LCD screen and the MediaTek Helio G35 SoC and supports 33W charging.ncG1vNJzZmivp6x7sbTOp5yaqpWjrm%2BvzqZmp52nqHynr8Jmm6ibpaKyr8CMrJ%2Bor6Niu6bE02amp52gocK0ec2oqZ1lp565rXnBnmSoqKCkeqKBlmZroJeZmX51fJRtaw%3D%3D