The guy contributed more to the world of comic books as a writer and editor than I can summarize without making this blog entry way too long. He wrote the classic socially-conscious Green Lantern/Green Arrow stories, gave Frank Miller how break on Daredevil, completely overhauled the Question into a great character and came up with the name Optimus Prime.
But the single biggest thing he did was make Batman serious. In the early 1970s, he removed the more light-hearted and fantastic elements from Batman, making the stories more grim and gritty, as well as more street level. He redefined Batman into the damaged, obsessed character that he has been for going on fifty years.
His work on Batman has informed literally every single version of the character since then.
So, yeah, Denny O’Neil was a big deal.