Bill O'Brien is not a very good coach at all.
Whether it's the way he talks, his game plan that changes on a weekly basis, or the way he constantly struggles to make a decision on what he wants to do at quarterback, he's just always come across as somebody who's in over his head and isn't cut out to be a head coach in the NFL.
Some people like to point to his winning record in that span but I think that has a lot more to do with the AFC South being the worst division in football throughout his tenure.