RDF ontologies are rapidly increasing in number. We study the problem of integrating two RDF ontologies under a given set H of Horn clauses that specify semantic relationships between terms in the ontology, as well as under a given set of negative constraints. We formally define the notion of a witness to the integrability of two RDF ontologies under such constraints. A witness represents a way of integrating the ontologies together. We define a "minimal" witnesses and provide the polynomial CROW (Computing RDF Ontology Witness) algorithm to find a witness. We report on the performance of CROW both on DAML, SchemaWeb and OntoBroker ontologies as well as on synthetically generated data. The experiments show that CROW works very well on real-life ontologies and scales to massive ontologies.