Antonyms of opinion:
denial, dissent, doubt, infidelity, rejection, suspicion, disbelief, distrust, incredulity, misgiving, skepticism, unbelief, fact, truth, deadlock, draw, halt, stalemate, standoff, tie.
Examples of usage:
  1. He fought because it was considered the " thing to do," because he liked the people he had to live with, and because those people wouldn't have a good opinion of him if he didn't fight.
    "Way of a Rebel", Walter M. Miller.
  2. I'm somehow of opinion that it was meant for me.
    "The Domestic Cat", Gordon Stables.
  3. " Matter of opinion," replied Ingleborough, " but here goes!"
    "A Dash from Diamond City", George Manville Fenn.