I am using an Oppo f3 plus smartphone and I cannot upgrade to Android 8.0, I am currently using Android 6.1
Maybe you can try your luck with custom ROMS, but it might not be easy if you haven't done this before.
I am utilizing an Oppo f3 in addition to cell phone and I can't move up to Android 8.0, I am at present utilizing Android 6.1
So how is it going now? Have you tried any custom ROMs?
You should contact to the Oppo customer care. They will resolve your problem and suggest you better as per your cell phone and other procedure.
You should go to the store to ask an expert who would be better
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