Ensuring the quality of Android apps presents a persistent challenge; more than half of the installed apps are uninstalled within 30 days of being downloaded because of poor quality. Nevertheless, functional testing of mobile apps is challenging because of the sheer variety of devices, operating systems, and network conditions. This paper broadens the scope of functional testing of Android apps from “in vitro” to “in vivo” on real-user devices. The proposed approach (O-MAT) leverages the device’s native resources to execute functional tests automatically on-device independently of external connections for test management. Compared to the emulator and client-server methods, the O-MAT approach is evaluated on nine open-source Android apps (on three diverse domains). We observe an average test execution time improvement of ~7% concerning the emulator and ~20% with respect to the client-server approaches for the overall nine Android apps.