For firms that own or sponsor a computer or smartphone standard, is it better to make the standard open or closed? Has openness paid as a business strategy? In this paper, the authors examine the history of operating systems in computing and mobile phones, and rely on four different notions of openness: open systems, open innovation, open-source software, and open governance. The authors conclude that the truly successful operating systems have been those whose owner or sponsor has managed to combine some degree of openness with some measure of control.