All that is true, but one thing: Each season had 13, not 15, episodes a piece.
As I've stated before, this is a subject I feel strongly about. Aside from being historically important in the history of anime in the west (first anime where all foreign audiences carried the weight for the second season, thus proving that we were more than a few extra bucks), the Big O was the first 'mature' anime I'd ever seen. I really want Chiaka Konaka to unveil the key to the mysteries once and for all.
If not a third season, then in the name of god, Adult Swim, make a movie.