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.
The officially launched with Android 6.0 (Marshmallow) and ColorOS 3.0. While many users look for an official Android 8.0 (Oreo) update, Oppo did not release a formal Oreo upgrade for this specific model. Oppo F3 Update 8.0 Download