In this paper we present a description of an OWL Ontology of fundamental legal concepts developed within the ESTRELLA European project. The aim of the ontology is to contribute to enable semantic access to digital legal information and clarify the legal theoretical concepts such as deontic modalities, obligative rights, permissive rights, liberty rights, liability rights, different kinds of legal powers, potestative rights (rights to produce legal results) and sources of law. Besides the taxonomy the ontology comprises also the semantic relations between the above concepts.