Opportunistic Routing (OR) has recently been proposed to improve the efficiency of unicast in multi-hop wireless networks. OR exploits the broadcast nature of wireless transmission medium and opportunistically selects a relay path to deliver a packet to its receiver. To adopt OR in wireless multicast, the main challenge is to efficiently share opportunistic relay paths between multiple receivers. In this paper, we propose an opportunistic overlay multicast design for wireless networks, named Minimum Steiner Tree with Opportunistic Routing (MSTOR). In MSTOR, the source and receivers are connected by an overlay Steiner tree. The source multicasts packets along the overlay links of the Steiner tree to reach all receivers. The transmission of packets on each overlay link is controlled by unicast OR. We first propose an overlay construction algorithm based on the optimal "OR distance" between nodes. We then design the MSTOR protocol and implement it in OPNET by customizing the IEEE 802.15.4 modules. Through OPNET simulations, we study the performance improvement of MSTOR over several existing unicast and multicast routing schemes. Our results demonstrate that MSTOR can achieve a much higher multicast efficiency than the original unicast OR and the traditional minimum multicast-tree based schemes. MSTOR can be easily deployed for multicast in multi-hop wireless networks.