Yes, opinion is a noun. Use it whenever you name what someone thinks. What's your opinion on the matter?
Choosing your sentence shape
Naming someone's view as the subject: My opinion hasn't changed since last year. Here opinion is the thing the sentence is about, so it sits in subject position with my in front of it.
Naming what someone thinks as the object of a verb: She asked for my opinion on the draft. The verb asked for needs a thing to receive, and the noun supplies it.
After a preposition: In my opinion, the plan needs another week. The phrase in my opinion works because opinion can head a prepositional phrase, which nouns do.
Plural counting: We gathered opinions from the whole team. Because you can make it plural and count them, opinions behaves the way nouns behave.
When a noun modifies another noun, it stays a noun. In opinion poll, opinion sits before poll like an adjective would, but it's still describing as a noun does; opinion never turns into a describing word on its own.
Quick check while you write: if you can put my, the, or a number before it, you're using the noun. That's one opinion passes the test; a non-noun reading wouldn't.
The one qualifier that changes your wording here is the preposition after opinion. Both on and about fit, so pick one and stay with it in your sentence.
If you're asking someone directly for their view, use on: What's your opinion on the new schedule? This is the natural choice when the topic is named right after.
If you're giving your own view inside a longer sentence, about reads smoothly: She changed her opinion about working from home. The preposition links the noun back to the topic without adding anything.
Either works with the same noun, so the decision is stylistic, not grammatical. They're still forming an opinion on the proposal and They're still forming an opinion about the proposal both pass; repeating a different one later would sound off.