An opinion doesn't require evidence. I can have an opinion of something with no evidence whatsoever. My opinion may turn out to be completely wrong and all evidence can contradict it, but it can still be my opinion.
A theory, on the other hand, is pretty far along in the heirarchy of scientific inquiry. It cannot be a theory until it has survived a LOT of attempts to disprove it.
Even the lowly hypothesis, which is sort of the developmental stage of the theory (it's what you're going to keep revising based on incoming data until you can't find anything else wrong with it to revise, at which time you bash it around some more, and might make it a theory if you can't disprove it despite myriad efforts to do so) requires some observation to support it.
An opinion requires no evidence or observations whatsoever. For the sake of argument, I could hold the opinion that you're just the most horribly rude and selfish person ever. You could try to disprove me by telling me about all the homeless animals you've adopted, the organs you've donated to critically ill children, the trips you've taken to third world nations to provide medical care to the needy, and I can STILL hold the opinion you're horribly rude and selfish in spite of all evidence to the contrary. On the other hand, using scientific method, if that were my initial hypothesis (maybe you stepped on my toe and didn't apologize, and that was my observational basis for the hypothesis), after being presented with evidence to the contrary, I would need to revise my hypothesis that you were only rude when you stepped on people's toes. Upon further testing, I might find that more often than not, you did apologize if you stepped on someone's toes, and would need to revise further that you were only rude if you stepped on MY toes. And if you further proceeded to apologize every time after that when you stepped on my toes, I might have to concede that statistically, it was random chance that you didn't apologize that one time, and the bulk of evidence doesn't support my hypothesis at all. But I could still stick to my opinion if I wanted, even if my opinion was totally unsupported and wrong.
