He didn't do a good job utilizing Webber and AI. He did a bad job actually. He didn't want Webber, he wanted Walker and when he didn't get him he pretty much acted like a baby. He put in no new plays to involve Webber until he was approached by AI and other players and asked to do so.
He had problems with a number of players, mainly Dalembert, who refused to resign if O'Brien was there and Webber who made it clear he didn't want to play for O'Brien. O'Brien also had problems with the front office due to his attitude about getting the players he wanted and playing time with some of the young players.
He also is a decent coach, but a stubborn one. He did a poor job of managing his players and adapting to them, instead trying to force them into his old Celts team. He also was a poor late game coach, something that Mo is as well.
O'Brien was better than Mo, not by that much, but there were too many problems surrounding him to keep him. But it was another f-up by the front office.