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. In this paper, we explore the gain of adopting OR in wireless multicast. The main challenge is to efficiently share opportunistic relay paths between multiple receivers. We proposed a scheme in  named Minimum Steiner Tree with Opportunistic Routing (MSTOR). In MSTOR, the source and receivers are connected by a Steiner tree. Packets are transmitted using Unicast OR on each link of the tree. MSTOR does not exploit opportunistic receptions cross different links in the tree. In this paper, we propose a Multicast OR (MOR) algorithm follow another approach. Starting from the source, MOR opportunistically employs a set of forwarders to push a packet closer to all receivers round-by-round. Each forwarder is responsible for sending the packet to a subset of receivers, and transmits the packet once each round. Based on packet receptions at the end of each round, a new forwarder set is constructed to maximize the Expect Transmission Advancement (ETA) towards all receivers. We study the performance improvement of MSTOR and MOR using numerical simulations. Our results demonstrate that both MOR and MSTOR can achieve a much higher multicast efficiency than the original Unicast OR and the traditional minimum multicast-tree based schemes. MOR constantly achieves significant transmission cost savings in all studied cases.