This paper presents OBAMAS (Ontology-Based Approach for Multi-Agent Systems engineering), an ontology-based contribution to Agent Oriented Software Engineering. We propose a formal process for agentification, starting with an analysis phase which consists of the construction of three formal ontologies (a domain ontology, an ontology of functionalities, and an ontology of multi-agents systems) and their alignment to merge in a single one. The second step, which is a design phase, consists of the operationalization of the single ontology in order to infer in a more formal way the agents of the system. A case study is introduced to illustrate OBAMAS and to show its use and effectiveness in a real application, a distance learning system.