Hardware/driver limitations with MediaTek MT6750T and Oppo’s prioritization of newer models (F7, F9, Find X).

If you are determined to run Android 8.0 or higher, you must use Custom ROMs . This process is risky and intended for advanced users: OPPO Software Update : Step-by Step Guide

This is a generalized guide for advanced users. If you are uncomfortable with command-line tools, do not proceed.