If you absolutely need Android 8.0+ features (picture-in-picture, notification channels, better background limits), it is time to consider upgrading your device. The Oppo F3 had a great run, but its software journey ended with Nougat.
Even if Android 8.0 isn't available, you should ensure you are on the highest possible official version for security and performance. How to check & update your Android version Oppo F3 Update 8.0 Download