The Operating System (OS) manages the hardware resources of a computer. For an OS, the knowledge about context is valuable information in optimizing its tasks. Recent mobile devices, such as smart-phones and tablets, are providing new avenues in context aware computing, because of the wide variety of sensors integrated into them. This paper identifies the importance of integrating context awareness at the OS level. In particular, we propose ContextOS, a proof of concept implementation of a context aware OS for mobile devices. ContextOS leverages the context knowledge for process scheduling. We implemented ContextOS in Android. The results of our experiments confirm the feasibility and usability of our solution.