The news broke last night on Twitter, and O'Donnell characterized the departure as not exactly amicable.
I'm saddened to say that Bungie's board of directors terminated me without cause on April 11, 2014.— Marty O'Donnell (@MartyTheElder) April 16, 2014
Bungie has not publicly commented on the nature of why O'Donnell has left, but the company did publish a brief blog post entitled "there are those who said this day would never come..." that wished him well.
"For more than a decade, Marty O’Donnell filled our worlds with unforgettable sounds and soundtracks, and left an indelible mark on our fans," said the company in a statement. "Today, as friends, we say goodbye. We know that wherever his journey takes him, he will always have a bright and hopeful future. We wish him luck in all his future endeavors."
O'Donnell was responsible for composing the Halo games, a series with one of gaming's most memorable modern themes. He was also scoring the studio's upcoming Destiny, which included a collaboration with Paul McCartney. Destiny isn't scheduled to be released until September 9.