With the emergence of the Industrial Internet of Things (IIoT), a huge amount of devices in the factory need be connected to the Internet. The use cases being considered include data exchange for inter-factory manufacturing, integrity of data collection and real-time monitoring, etc. It is important to allow seamless communications among IIoT devices of different protocols and standards. In this research, we investigate how to achieve IIoT interoperability via two popular global IIoT standards: oneM2M and OPC-UA by adopting the OPC-UA system in the field domain but connecting it to the oneM2M system in the infrastructure domain. We propose an optimized design of oneM2M Interworking Proxy Entity for resources mapping and procedures mapping between OPC-UA and oneM2M. The design contains both (1) interworking functions and (2) OPC-UA client APP, installed on the oneM2M Middle Node in the field domain, to handle the data exchange.