Continuous and unforeseeable evolution of business rules, processing logics, and organizational structures within enterprises, requires from business process management systems to integrate continuous design. Supporting business process rediscovery based on workflow logs analysis, workflow mining gathers retroactive (re)design techniques necessary to understand business process execution reality. Most of the works in this area focus on the control flow perspective, while very few of them address the organizational aspect. In addition, the used workflow mining techniques suffer from the lack of automation due to the purely syntactic logs. In this paper, we propose an Organizational Ontology (OrO) specifying the organizational model. This ontology is used to semantically annotate log files and establish an organizational knowledge base that will be used to discover relationships between performers in a workflow. Our approach has been implemented within the ProM framework