You'll need to use a noun, noun phrase or noun clause: What is your opinion of/about junk food? What is your opinion of/about the sale of junk food to children? What is your opinion of/about the way the convenience stores all sell junk food to children?
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AnonymousWhat is your opinion of/about ...?I use them in almost the same way. "about" is more common and more useful and can include just about anything. I probably use "of" only with people or things that people have produced if I use it at all.