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.
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.
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