Nouns ending in -on are usually masculine, but l'opinion is feminine.
Opinion is one of the rare exceptions: learn it as it is.
Is opinion masculine or feminine?
l'opinionexception
The rule behind it
91.9% of -on nouns are masculine
In context
Ce sont des opinions tout à fait opposées.
These are completely different opinions.
Je suis indifférent à l'opinion des autres.
I am indifferent to others' opinions.
Mon opinion est entièrement différente de la vôtre.
My opinion is entirely different from yours.