I think the word which best complements the word "opinion" is "belief:"
conviction of the truth of some statement or the reality of some being
or phenomenon especially when based on examination of evidence.
An opinion is a way that someone feels about something, and a belief is something that some person (or some animal, I guess) thinks is true. They're both the same in that people have them.
However, based on the way you worded your question, it seems like you're just looking for a word which refers to an objective statement without calling that statement a fact. In that case you could use "proposition:"
An expression in language or signs of something that can be believed,
doubted, or denied or is either true or false
or maybe "hypothesis."
A possible adjective to describe statements which are either true or false is "Objective:"
Expressing or dealing with facts or conditions as perceived without
distortion by personal feelings, prejudices, or interpretations.
Another word, "empirical:"
Capable of being verified or disproved by observation or experiment
I don't really like here, because in my experience, "empirical" is typically used to refer to a proposition which can not be reasoned about (i.e. requires data collection).
I think "verifiable:"
capable of being verified
has a similar problem, because there are statements which are objective but not verifiable (i.e. "God exists.")