We examine the decoherence properties of a quantum open system as modeled by a quantum optical system in the Markov regime. We look for decoherence in both the environment-induced superselection (EIS) and consistent histories frameworks. We propose a general measure of the coherence of the reduced density matrix and find that EIS decoherence occurs in a number of bases for this model. The degree of "diagonality" achieved increases with bath temperature. We evaluate the decoherence functional of consistent histories for coarse-grained phase-space two-time projected histories. Using the measures proposed by Dowker and Halliwell we find that the consistency of the histories improves with increasing bath temperature, time, and final grain size and decreases with initial grain size. The peaking increases with increasing grain size and decreases with increasing bath temperature. Adopting the above-proposed measure of "coherence" to the decoherence functional gives similar results. The results agree in general with expectations while the anomalous dependence of the consistency on the initial grain size is discussed.