Oppo devices traditionally have locked bootloaders. You will need to unlock it, which typically wipes all data.
Factory resetting the device via software. Remove FRP Lock: Removing Google Factory Reset Protection. Downgrade or Upgrade: Returning to a stable stock version. Oppo F1s Firmware Download 7-1-2
Resolving issues with apps crashing or system UI errors. Pre-requisites Before Downloading Oppo devices traditionally have locked bootloaders
Oppo does not host public firmware on their main website. Instead, they provide tools like the feature. However, for offline flashing, you can find official stock ROMs from verified community mirrors. for offline flashing
Official support ended after an upgrade to Android 6.0 Marshmallow.
The phone's bootloader must be unlocked to allow custom software.