"You can experience a download, but you can't download an experience."
- Billy Bragg
Some unofficial tools and exploits exist, particularly for MediaTek-based OPPO devices , though the A9 2020 uses a Snapdragon chip, making it even more restrictive.
The primary reason custom ROMs are rare for the Oppo A9 2020 is that OPPO does not officially support bootloader unlocking . Without an unlocked bootloader, you cannot install a custom recovery (like TWRP) or flash a new operating system.
OPPO has made it nearly impossible to unlock the bootloader on recent models through standard developer options.
Unlocking Potential: Does a Custom ROM Work on the Oppo A9 2020?