Ontology-based data access (OBDA) has become a popular paradigm for accessing data stored in legacy sources using Semantic Web technologies. In the OBDA setting, users access the data through a conceptual layer, which provides a convenient query vocabulary abstracting from specific aspects related to the data sources. This conceptual layer is typically expressed as an RDF(S) or OWL ontology, and it is connected to the underlying relational databases using R2RML mappings. When the ontology is queried in SPARQL, the OBDA system exploits the mappings to retrieve elements from the data sources and construct the answers expected by the user. Different approaches for query processing in OBDA have been proposed. We focus here on the virtual approach, which avoids materializing triples retrieved through mappings and answers the SPARQL queries by translating them into SQL queries over the data sources. In this paper we present our mature open-source OBDA framework Ontop, which supports all W3C standards related to OBDA, and which produces efficiently executable SQL queries.