In order to enable the development of smart services, many IoT platforms were proposed to securely and reliably connect IoT devices to the Internet and to each other. Examples are IoTivity from Open Connectivity Foundation (OCF), AllJoyn from AllSeen Alliance, Weave from Google and HomeKit from Apple. However, such fragmentation of IoT platforms has made the development of IoT applications very difficult as it requires each IoT device and application support many protocols in order to connect to an available platform. To resolve this problem, we propose to use oneM2M International standards as the middleware for IoT protocols integration. The oneM2M technical specifications have defined how oneM2M can be used to interwork with IoTivity and AllJoyn based on Interworking Proxy Application Entity (IPE). In this paper, we propose how to design and implement such an IPE based on the oneM2M specifications for bridging oneM2M and IoTivity/AllJoyn. In particular, we address the mappings of device management functions among these platforms where standard specifications are still lacking. Via actual implementation, we evaluate two interworking designs for device management in order to identify the best one for protocol integration.