Connect your Oppo A57 in EDL mode. The tool should show "Qualcomm HS-USB QDLoader 9008" at the top.
Firehose file, you typically need specific tools and environment settings: oppo a57 firehose file
Searching for “Oppo A57 Firehose file” leads you to sketchy file-hosting sites (4shared, Mega, random Russian forums). Here is your safety checklist: Connect your Oppo A57 in EDL mode
: Installing or repairing the device's Stock ROM at a system level. Model Identification: CPH1701 vs. CPH2387 oppo a57 firehose file
buttons simultaneously while connecting the phone to your PC via a USB cable.