Optimistic Atomic Broadcast F.Pedone and A.Schiper This paper presents an Optimistic Atomic Broadcast algorithm (OPT-ABcast) that exploits the spontaneous total order message reception property experienced in local area networks, in order to allow fast delivery of messages. The OPT-ABcast algorithm is based on the Optimistic Consensus problem (OPT-Consensus) that allows processes to decide optimistically or conservatively. A process optimistically decides if it knows that the spontaneous total order message reception property holds, otherwise it decides conservatively. We evaluate the efficiency of the OPT-ABcast and the OPT-Consensus algorithms using the notion of latency degree.