Flashing custom ROMs voids your warranty, wipes all your data, and can potentially brick (permanently damage) your device if done incorrectly. The author and site are not responsible for any damage. Proceed at your own risk.
Since OPPO locks bootloaders tightly, follow this community-developed method: oppo a5s custom rom download
Search for "TWRP Realme C2" (or build from source for CPH1909). Installation via Fastboot: Flashing custom ROMs voids your warranty, wipes all