I suppose if someone were truly humble then he or she wouldn’t really have an opinion and just agree with what others say. 
Humility to me means an honest assessment, usually of one’s self. It would be dishonest of me to say that I lacked expertise in certain areas where I am frankly paid to be an expert.
Having one, no. Saying you have one, yes.
A “humble opinion” can be a perfectly nonoxymoronic* thing. “Humble” does not mean “pushover.” Any opinion that is honest and that is not offered to create any reaction in particular is a humble opinion, in my humble opinion.
*In greek, “oxy” means “sharp.” I’m curious what “blunt” or “flat” is. Anybody know?
Moron.
If anything, it’s more redundant than oxymoronic. To claim an opinion as fact is arrogant; to qualify a claim as only an opinion is humble. “IMHO” is just an intensified form of “IMO”.
Not an oxymoron.* A humble opinion is one given without bragging; it’s a sign of humility. Of course, nowadays, that’s a false humility, but that doesn’t matter. The idea of being humble and also having an opinion are far from contradictory. At most, it’s an ironic use of “humble,” but an oxymoron would be “humble bragging,” not “humble opinion” (an opinion isn’t bragging).
*Neither is “jumbo shrimp,” BTW – a shrimp is a crustacean that can be a variety of sizes.
Swiss cheese
A humble person can have and express an opinion. You don’t lose the humility until you start claiming your opinion is right and other people’s opinions are wrong.
Not an oxymoron, just mostly used dishonestly.
Yes, I can see how the phrase can be redundant, but I was more interested in the implied meaning of “humble” or its connotation as being agreeable, submissive, meek, or silent to the opinions of others. Clearly, if someone were humble in every sense of the word, then that person would just nod his head in agreement to all that has been said.
Anyway, I was being a bit facetious in my question, as I sometimes want to tell people that if they were truly humble then they wouldn’t have an opinion and just agree with me. 
Whuh? Those aren’t the qualities that “humble” brings to my mind. Sure, being humble is incompatible with being arrogant, or boastful, but I’ve never gotten the impression that it means being the opposite of those. Certainly not to the point of pretending not to have a different opinion from anyone.
“Humble” in this context sort of brings a (movie-land) buddhist monk sort of thing to mind for me. Somebody very wise that will listen to you run your mouth off, but rather than telling you off, will offer you his statement and let it drop quietly. The way philosophers (or even Jesus) are often portrayed – the common story where two merchants are running their mouths off screaming at each other and ask the philosopher to resolve their dispute, and the philosopher lets loose two sentences that politely, elegantly, and succinctly show why both of them are wrong, as well as introducing a viewpoint neither have considered, and even if the merchants argue back at the philosopher, all he does is calmly raise another point in response. Obviously the “actually being correct” part isn’t entailed in the humbleness, just the demeanor.
A “humble” opinion doesn’t even necessarily have to even consider the other person’s view as valid, you may think their opinion on the matter is exceedingly poorly informed, bordering on stupid. But the “humbleness” reflected is that outward presence of calm. That sense of allowing others to speak their mind, and calmly offering rebuttals and pieces of mind with no outward fanfare or obvious pride.
“In my humble opinion” isn’t the sentiment that can be rephrased “I am humble; here is my opinion”. That would be the first definition of humble: meek, submissive, deferential.
It is the other definition of “humble”: insignificant, unworthy, lowly, and it refers to the opinion rather than the speaker. You could rephrase it “My opinion is poor, inferior and unworthy; nevertheless here it is”.
It’s actually a facetious way of saying it–or it was, before it became an abbreviation.
I agree. Surely it’s the opinion that’s being described as humble, not the person who has it.
Wrong. It doesn’t make you an arrogant bastard just because you thought the spaghetti sauce was pretty good.
I’d say it could be used both reasonably and sincerely in several senses of the word.
“Humble” meaning–
–courteous and respectful: “You have an excellent point, but for me, personally, is a more important factor.”
–modest; aware of one’s own failings: “I don’t really know much about this, but I think…”
–deferential: “You know more about this than I do, but from my layman’s perspective, it seems…” (or “It’s your decision, so your opinion is the important one, but since you asked…”)
I just read in a post that someone said someone else had “correct opinions.” Is that a possible thing?
Only my opinions are correct. You can share my correct opinions, but mine are all correct by definition.