opinion vs supposition

opinion and supposition both are nouns.

Word	Noun	Adjective	Verb	Adverb
opinion	Yes	No	No	No
supposition	Yes	No	No	No

As nouns, supposition is a hyponym of opinion; that is, supposition is a word with a more specific, narrower meaning than opinion:

opinion: a message expressing a belief about something; the expression of a belief that is held with confidence but not substantiated by positive knowledge or proof
supposition: a message expressing an opinion based on incomplete evidence

Other hyponyms of opinion include adverse opinion, conjecture, guess, hypothesis, speculation, surmisal, surmise, position, side.

opinion (noun)	supposition (noun)
the reason for a court's judgment (as opposed to the decision itself)	a hypothesis that is taken for granted
a personal belief or judgment that is not founded on proof or certainty	the cognitive process of supposing
a belief or sentiment shared by most people; the voice of the people	a message expressing an opinion based on incomplete evidence
a vague idea in which some confidence is placed
the legal document stating the reasons for a judicial decision
a message expressing a belief about something; the expression of a belief that is held with confidence but not substantiated by positive knowledge or proof