The oneM2M global initiative has been working to standardize horizontal machine-to-machine (M2M) and the Internet of Things (IoT) service layer specifications for globally interoperable M2M/IoT solutions. However, there exist a myriad of embedded sensor systems not complying with oneM2M, such as de facto IoT standards (e.g., AllJoyn and IoTivity). In this paper, we present our effort in global interworking of IoT systems across multiple M2M/IoT standards, in particular, on a global scale. By implementing specialized oneM2M entities for interworking, interworking proxy entities (IPEs), we demonstrate a promise of globally interoperable IoT systems interconnected with all kinds of embedded systems.