Web Usage Mining (WUM) aims to extract navigation usage patterns from Web server logs. Mining algorithms yield usage patterns, but finding the ones that constitute new and interesting knowledge in the domain remains a challenge. Typically, analysts have to deal with a huge volume of pattern, from which they have to retrieve the potentially interesting one and interpret what they reveal about the domain. In this paper, we discuss the filtering mechanisms of O3R, an environment supporting the retrieval and interpretation of sequential navigation patterns. All O3R functionality is based on the availability of the domain ontology, which dynamically provides meaning to URLs. The analyst uses ontology concepts to define filters, which can be applied according to two filtering mechanisms: equivalence and similarity.