There is no official Android 8.0 (Oreo) update released by OPPO for the F3 model. If you are looking to move beyond Android 7.1.1, you must use unofficial third-party software (Custom ROMs), which comes with significant risks. Official Update Path (Up to Android 7.1.1)
If available, download the firmware (.zip) to your phone's storage. Power off the phone, then hold Power + Volume Down to enter Recovery Mode.