Firmware is the low-level software embedded in your phone’s hardware. For the OPPO A18, the firmware acts as the bridge between the hardware components (processor, camera, battery) and the ColorOS user interface. Without correct firmware, your phone is essentially a brick.
Due to the entry-level nature of the OPPO A18 and MediaTek’s proprietary security (SP Flash Tool requirements), there is little to no custom ROM development (like LineageOS) for this model. Attempting to flash cross-model firmware will hard-brick the device. Firmware OPPO A18