Message ordering is one of the cornerstones of reliable distributed systems. However, some ordering guarantees, such as atomic order, are expensive to implement in terms of message delays. This paper presents Optimistic Atomic Multicast, a protocol that combines reduced latency and increased throughput. Messages can be delivered optimistically in a single communication step and conservatively in three communication steps. Differently from previous optimistic group communication protocols, Optimistic Atomic Multicast does not rely on spontaneous message ordering for fast delivery. In addition to presenting Optimistic Atomic Multicast, we provide detailed performance results comparing it to other ordering protocols in both local-area and wide-area networks.