Opportunistic routing (OR) involves multiple forwarding candidates to relay packets by taking advantage of the broadcast nature and multi-user diversity of the wireless medium. Compared with Traditional Routing (TR), OR is more suitable for the unreliable wireless link, and can evidently improve the end to end throughput of Wireless Mesh Networks (WMNs). At present, there are many achievements concerning OR in the single radio wireless network. However, the study of OR in multi radio wireless network stays the beginning stage. In this paper, we focus on OR in multi-radio multi-channel WMNs. We validate the advantage of OR in multi-radio multi-channel WMNs, and propose a Simple Channel Assignment for Opportunistic Routing (SCAOR), which assigns channel to flows. According to interference state of every node, SCAOR assigns a channel with minimum interference to each flow to balance channel load. The simulation result shows OR of dual-radio dual-channel WMNs can promote throughput evidently, specifically, 16.8% higher than throughput of TR in the dual-radio dual-channel WMNs, 87.11% and 111.8% higher than throughput of OR and TR in single-radio single-channel, respectively.