Converting an (specifically the older China variants like the A57t ) to global firmware is a popular procedure to gain access to Google Play Services and remove Chinese "bloatware." While technically possible through specialized tools, it carries significant risks, including potentially "bricking" the device if done incorrectly. Core Requirements for Conversion
Assuming you successfully manage to flash Global firmware or a custom ROM (via TWRP, if available for your specific security patch oppo a57 china to global firmware