Downloading the correct file is often the hardest part. Many websites offer "Oppo A3s Firehose" files, but they might be corrupted, infected with malware, or simply incompatible with your specific region variant.
Here are the requirements before you download:
Oppo provides an official tool for authorized service centers. The MSM tool includes the Firehose internally.
Note: This tool will wipe all data and relock the bootloader.
Ensure you have installed the Qualcomm USB Driver on your PC. Without this, the computer will not recognize the phone when it enters EDL mode.
Since the Oppo A3s does not have a dedicated EDL mode button combination in the stock recovery, you have two options:
The device should now appear in Device Manager under Ports (COM & LPT) as Qualcomm HS-USB QDLoader 9008.