I am using an Oppo f3 plus smartphone and I cannot upgrade to Android 8.0, I am currently using Android 6.1
Please contact to OPPO directly as it is up to OPPO whether the new Android 8.0 ROM is available/suitable for the F3 Plus smart phone or not.
I did a update to my phone since then he didn't get on when i try to contact oppo they always say that they are not responsible
Yep, you are right