Socrates (as represented in Plato's Meno) suggests that three kinds of understanding may provide the basis for thinking that something is true: actual knowledge, right (or true) opinion, and wrong (or false) opinion. Here is an excerpt from that dialogue:
Socrates. I will explain. If a man knew the way to Larisa, or anywhere else, and went to the place and led others thither, would he not be a right and good guide?
Socrates. And a person who had a right opinion about the way, but had never been and did not know, might be a good guide also, might he not?
Socrates. And while he has true opinion about that which the other knows, he will be just as good a guide if he thinks the truth, as he who knows the truth?
Socrates. Then true opinion is as good a guide to correct action as knowledge; and that was the point which we omitted in our speculation about the nature of virtue, when we said that knowledge only is the guide of right action; whereas there is also right opinion.
Socrates. Then right opinion is not less useful than knowledge?
Meno. The difference, Socrates, is only that he who has knowledge will always be right; but he who has right opinion will sometimes be right, and sometimes not.
Meno makes a logical misstep here. He is thinking about how some opinions are right and some are wrong, but he doesn't express himself in those terms. Instead, he follows Socrates' lead in focusing on right opinion to the exclusion of considering wrong opinion at all. But if an opinion is right, as Socrates later points out, it is functionally indistinguishable from knowledge—because, like knowledge, it leads to a correct result.
Of course, narrowing the focus from "right or wrong opinion" to "right opinion" begs the question of whether one has any basis for deciding whether an opinion is right or wrong unless one has access to actual knowledge to determine the opinion's validity. Clearly, "right opinion" would be a redundancy if "opinion" could not, in some instances, be wrong. But because Socrates' interest in this dialogue lies elsewhere—namely, in the argument that people can have a true opinion about what virtue is even if they can't claim to have absolute knowledge of it—he is perfectly happy to ignore the issue of "wrong opinion" altogether.
Nevertheless, by the terms of his argument, Socrates seems to accept the existence of three possible bases for belief in the truth of a particular proposition: knowledge, which is by definition true; right (or true) opinion, which is, by dumb luck or the grace of God, correct and thus indistinguishable in its effects from knowledge; and wrong (or false) opinion, which tends to lead its possessor away from the truth.
In all of these considerations, Socrates seems to take the view that a person's opinions may be right (true) and lead toward a correct understanding of reality, or wrong (false) and lead toward an incorrect understanding of reality. If we accept the implicit definition of "opinion" as a view of what is actually so in a particular instance, it seems clear that an opinion can be right or wrong.