Important notice for Chinese OnePlus 15 devices running OxygenOS with a locked bootloader
If you have a Chinese OnePlus 15 that was converted to OxygenOS and the bootloader was locked again, I strongly recommend restoring bootloader/Fastboot access now while the free method is still available.
You do not need to unlock the bootloader afterward. The important part is restoring access to the real bootloader/Fastboot mode.
If your device is affected, do this sooner rather than later.
Huge thanks to mikoker for discovering this method.