Many users search for "Firmware OPPO A57s" hoping to find custom ROMs like LineageOS or Pixel Experience. The OPPO A57s has a locked bootloader , and official methods to unlock it do not exist for this model. Mediatek devices (which the A57s uses with a Helio G35) are difficult to unlock.
Users search for firmware files for several reasons. Here are the most common scenarios: Firmware OPPO A57s
Stick to stock firmware.