ITT: Not defining your terms from the get go and then end up debating it for a large portion of the thread
Seriously, it wasnt a good idea to not define what you mean by opinion. We could be talking about entirely differ ideas (and we have been) and then end up arguing fruitlessly about them (and we have been).
An opinion in my definition is something which is inherently subjective, like "strawberry flavored ice cream is better than vanilla flavored ice cream." You can argue all you want, but at the end of the day, you may never convince them otherwise.
However, if their opinion has facts as the underlying basis for it, you still may be able to. For example, someone could say that, in their opinion, X movie is good. If they base this with things like dialogue, acting, etc, you can try to convince them that the dialogue could have been better, or the actors shouldn't have filled the roll they did. They still might think the movie is good – heck, maybe they like it for the very reasons you dislike it – but they might not as well.