You - and many answers - are assuming that "fact" and "opinion" are two distinct categories. They are not. One is not the opposite of the other and they are not exclusive. Most examples given here are overly simplified and don't do that justice.
In many longer discussions about what a "fact" actually is - such as this one - the word "opinion" never appears.
A fact is generally considered to be something "in the world", objective, measurable in some way, something for which a truth value can be established, at least in theory. Think "fact-checking".
Whereas an opinion is a mental state. A belief someone has about something. That belief can be identical to a fact. In a murder case, you can have an opinion about whether the accused is guilty or not. That opinion may or may not align with the facts (and we may or may not actually prove the facts).
Note that most everyday speech is shortened, with implied content that isn't spoken. When someone says "it is hot", the full statement, if you could convince them to state in full the meaning of it, would be something like "I feel that the temperature today is well above what I consider average." - and spelled out in full, this is getting close to something that could be factually checked. However, that isn't the difference. Many opinions could be fact-checked. What makes them opinions is that they are a personal, internal experience.
So you have the "fact" - part of the world - and the "opinion" - part of the subjective experience. About the first we can argue, observe, measure. About the second we cannot - that I disagree with your opinion is simply a different opinion, but there is nothing exclusive about it. Either, neither or both of our opinions could be true-to-facts as well, or be unknown or undeterminable with regards to that. A specific statement can be both fact and opinion. If the distinction is important, you need to make it explicit. Language being the murky thing it is doesn't always do that automatically.