Not everyone. Just the bureaucrats, politicians, and newscasters. Ordinary people could still call her "Oprah" as long as it wasn't a formal meeting or somesuch.Oprah could never be president because then she'd have to be called 'President Winfrey' by everyone and forego universal recognition as just 'Oprah'. Her first name is the brand, that's the power word; she'd be mad to give it up.
Here in Canada, if someone refers to "Justin" there are two choices as to which person is being talked about: Teenagers would probably think it was the "singer" (honestly, any country that wants him can have him), and anyone older would likely think it was the Prime Minister.