An opinion was an individual's view, belief, or judgment about something. Being personal, subjective, and subject to change, an opinion could not be proved by evidence. The basis for the opinion came from the individual's thoughts, feelings, and understanding. When offering an opinion, an individual might add facts to lend credibility to what they were saying. Within a society which valued individuality, opinions could differ from person to person. (TOS: "The Changeling")
In 2254, after receiving a distress signal from the SS Columbia, Captain Christopher Pike weighed his options. Would he continue on to the Vega colony, where crewmembers recently injured in a battle on Rigel VII would be treated, or would he respond to the signal, helping any survivors from the crashed survey vessel? As the vessel crashed eighteen years ago, and the probability of there being survivors was low, Pike chose the former option, placing his crew first. He asked for the opinion of the ship's Doctor, Doctor Phil Boyce, on the matter, saying that it would be entered into the ship's log. Boyce responded with his opinion, saying "Oh, I concur with yours, definitely." (TOS: "The Cage", "The Menagerie, Part I")
In 2364, Captain Jean-Luc Picard asked Counselor Deanna Troi's opinion on whether the Ligonians would injure the kidnapped Tasha Yar. Troi didn't believe so as the Ligonians seemed mainly curious, however, she did sense some sexual attraction from all the males. (TNG: "Code of Honor")