Note: change to slot-a when done then reboot [ fastboot --set-active=a ]
Linux (in the example below I will use ubuntu)
Install adb & fastboot and simg2img
sudo apt-get install android-tools-adb android-tools-fastboot simg2img
then run the program with
./OppoRealme-OFP-Flash_1.0.bin
Windows:
Extract to a folder and run the program.
macOS:
Install adb & fastboot and simg2img
then run the program with
./OppoRealme-OFP-Flash_1.0.bin