In this paper we present an extension to the OLSR unicast routing protocol to support multicast routing in mobile ad hoc networks. The proposed protocol is based on Multicast Overlay Spanning Trees (MOST). The main benefits of this approach are twofold. Firstly, it implies that only nodes interested in taking part in the multicast communication need to participate in the protocol operation, which is transparent to other OLSR nodes. In addition, the MOST approach scaling properties achieve the theoretical performance bounds concerning the capacity of multicast communication in massive ad hoc networks. We perform simulations of the MOST protocol under the ns-2 simulator to compare with the theoretical results, and we present a fully working implementation for real network environments.