O'Brien was a slight upgrade over Todd, yes, though both would have been solidly in the second-tier of QBs in the '80s. Behind Montana, Theismann, Marino, Elway, Moon, and I'd even include Kosar as top-tier elite in the '80s.
Kosar is easily forgotten but he was good. He was a stupid fumble and a miracle drive away from taking that team to a Super Bowl that they would have been blown out in LOL.