Ontology languages and corresponding query languages play key roles for representing and processing information about the real world for the emerging Semantic Web. Efforts have been made to develop various ontology languages. Each ontology language provides different expressive power and also computational complexity for reasoning. Ontology query languages were developed to query the information defined by these ontology languages and reasoning systems. We conduct a study to compare their expressive power, efficiency, scalability and best performing situation. We also introduce the OPS system which consists of two subsystems: the ROPS subsystem, an OWL reasoner built on Jena2 and the SWOPS subsystem, a First Order Logic (FOL) reasoning system based on Vampire. Each part can work separately and cooperate with each other for different tasks. In this paper, we will compare different ontology query languages together with query systems and give evaluations from the user s view point.