Back To Stock fastboot flashable files of Oxygen OS for OnePlus 7 Pro GM21AA variant
you will need 7zip for windows or p7zip for linux to extract the items. you will need an unlocked bootloader to flash. if you have windows, run the flash-all-partitions.bat and follow the prompts. bat files in the folder are made by Mauronofrio on XDA. feel free to give him thanks. i personally made up a .sh file for linux users but i used contents from one of the .bat files from Mauronofrio.