"opine on" is correct and usable in written English.
It means to express an opinion on something. For example, "She opined on the recent election results and their implications for the country."
THE New York Times's theater critics are not the only ones willing to opine on spring theater highlights.
At least one candidate was moved to opine on Exxon's earnings.
An election is one of the few subjects pundits opine on en masse for which there is an unambiguous outcome.
When pundits opine on the difficulty of difference, they flatten identity into a single image, just one experience.
He declined to opine on them.
My job as speaker is to opine on legislation.
I don't opine on things I haven't studied.
And I guess commentators can -- can opine on it.
The Court did not opine on the validity of the bylaw under equitable principles.
Juries believe every word uttered by "expert" witnesses who opine on defendants they have never met.
Mr. Phaup has the bona fides to opine on this topic.
