A performance comparison according to different number of wavelengths and topologies on OBS has been studied in this paper. Preemptive Channel Scheduling Algorithm (PCSA) has been used as reservation mechanism in OBS. In terms of performance criteria, loss rate in bytes, access delay and end-to-end delay are considered. A 2-state MMPP (Markov Modulated Poisson Process) traffic generator is used. Four different types of Mesh and Ring topologies are used. NS2 Network Simulation tool is used for our tests. In OBS algorithms, bursts are created using a hybrid model that takes into account both timeout and maximum length threshold mechanisms. In nodes, in order to satisfy QoS requirements, priority based queuing and Regulative Wavelength Grouping (RWG) are used. In priority based queuing, packets (bursts) are sent according to their priority order. In this study, the effects of generated traffic according to the topologies and the effects of increased number of wavelengths are shown by access delays. According to the simulation studies, the success of byte drop rate increases while the number of wavelengths increases. The results that obtained on mesh topologies are better than the results of ring topologies according to our simulation results