This is the first and most crucial step. Without an unlocked bootloader, you cannot modify the system partitions.
Team Win Recovery Project (TWRP) is a custom recovery tool used to "flash" (install) the ROM files. oppo r9s plus custom rom
Since you wiped the system, you cannot see the ROM file on your phone. This is the first and most crucial step
: Save all important files and secure your device’s original stock firmware as a fallback. : You will need a PC with ADB and Fastboot drivers installed. Step 1: Unlock the Bootloader oppo r9s plus custom rom