Don't bring a dictionary to a logic fight. Nothing in the OED definition of "opinion" conflicts with anything Hussar said at all.
If Xs are not necessarily Y, the only thing that follows is that if something's an X, it's possible that it could also be Y. It obviously doesn't follow either that no Xs are Y (mammals are not necessarily male, but some mammals are male) or that Xs should be Y (library patrons don't necessarily return their books on time, but they should). (It doesn't even follow that any Xs are not Y: it so happens that every thing I'm wearing now is blue, but this isn't a necessary truth.) In fact, if opinions were necessarily based on fact or knowledge, there would be much less point to saying that they should be so in the first place: why bother holding something to a standard it can't possibly fail to meet?
Really? When you, in your capacity as a mod, tell someone that their behavior here has crossed a line, I'd say that you're expressing your opinion. But I'd also say that you generally have excellent reasons for that opinion, and that you wouldn't be nearly so good a mod if you didn't.