O'Connor, who has served in the development of Halo 2 and 3 on the Xbox and Xbox 360 respectively, broke the news of his departure on Bungie's website.

"I am leaving you," O'Connor wrote. "I am off to work more closely with Microsoft on the Halo franchise, so I shan't be too far away, but I wanted to take this last chance, appropriately enough in a Weekly Update, to tell you a very fond thanks."

While O'Connor and Microsoft's internal studios continue work on the Halo franchise, Bungie—which achieved corporate independence from Microsoft late last year—is meanwhile working on undisclosed new properties while continuing to support Halo 3 (360) with downloadable content updates.

Though it is unclear what role Bungie will play in the development of upcoming Halo titles, the studio reiterated as late as September of last year that it was continuing to collaborate on a new Halo game with director Peter Jackson's Wingnut Studios, commonly referred to as Halo Chronicles.

Offshoot RTS Halo Wars, meanwhile, is under development at Ensemble Studios with limited input from Bungie.