Straight from the dictionary:
Opinion on is only moderately used. Your opinion on something describes your ideas or beliefs regarding a particular matter or topic. For example, “What’s your opinion on the new tax law?” Usually you’d use this phrase to ask for someone’s opinion on a broad topic where many different opinions can be made. It’s the phrasing that can open up debates.
Both opinion on and opinion about are similar because they refer to the qualities of a topic. The preposition on emphasizes the topic of the opinion a bit more than the opinion itself. For example, “Chris has a strong opinion on taxes.” Here, taxes is emphasized as a topic for an opinion.
Opinion about is the least common phrase among the three. It’s often used to express when someone doesn’t have an opinion, as in “I don’t have an opinion about this.”
The preposition about emphasizes the opinion itself a bit more than the topic of the opinion. “We have strong opinions about food safety,” emphasizes opinions rather than the topic of food safety.