Motivated by realistic sensor network scenarios that have fading environment and opportunistic transmission, we propose a novel routing protocol, OMR, which integrates routing and MAC protocols to improve the packet delivery ratio, reduce the packet delivery latency, and decrease the energy consumption in multi-hop wireless sensor networks. OMR chooses a packet's route at each hop after the transmission to that hop and the choice is based on the fact that intermediate nodes actually receive the packet and can forward the packet to any nodes at the next hop with high probability. The nodes coordinate to guarantee hop-based reliability while avoiding redundant retransmission by blindly flooding. The timeout and sink initiated retransmission mechanisms by intermediate nodes in OMR guarantee the end-to-end reliability. Piggyback acknowledgement shares the packet and acknowledgement information among neighbors to mitigate the effects of lossy and asymmetric links. The results of intensive simulation using TOSSIM show that OMR performs well compared with traditional routing protocols.