Relational Databases (RDBs) are used in most current enterprise environments to store and manage data. The semantics of the data is not explicitly encoded in the relational model, but implicitly at the application level. Ontologies and Semantic Web technologies provide explicit semantics that allows data to be shared and reused across application, enterprise, and community boundaries. Converting all relational data to RDF is often not feasible, therefore we adopt a mediation approach for ontology-based access to RDBs. Existing mapping approaches focus on read-only access via SPARQL or as Linked Data but other data access interfaces exist, including approaches for updating RDF data. In this paper we present OntoAccess, an extensible platform for ontology-based read and write access to existing relational data. It encapsulates the translation logic in the core layer that provides the foundation of an extensible set of data access interfaces in the interface layer. We further present the formal definition of our RDB-to-RDF mapping, the architecture of our mediator platform, and a performance evaluation of the prototype implementation.