I'm not trying to act like the Grammar Police, but what you have stated here is not an "opinion." It is a supposition. Speculation. It is not an "opinion."
This has got to be one of the most common mistakes in modern discourse. Example: "In my opinion, Saddam did not have weapons of mass destruction." No, sorry, that is not an opinion. An opinion would be "It is bad that Saddam had weapons of mass destruction."
Clarity of writing leads to clarity of thought, and vice versa.
Is that an opinion or a subjective attitude? Why does this expression have to be anything at all, especially the most common mistake? Assertion of an opinion as fact might be common, but hearing an opinion as a fact is equally common.