You typically need to unlock the bootloader and install a custom recovery (like TWRP) before you can manually flash a new OS.
If you require Android 8 or newer, you must use unofficial third-party software, such as (based on Android 8.0). Important Considerations: oppo f3 android 8 update download full
Some regions received a beta/limited rollout of Android 7.1 Nougat , but Android 8 was never officially provided. You typically need to unlock the bootloader and