OntoComP is a Protégé 4 plug-in for completing OWL ontologies. It enables the user to check whether an OWL ontology contains "all relevant information" about the application domain, and extends the ontology appropriately if this is not the case. It achieves this by asking the user questions of the form "are instances of classes C1 and C2 also instances of class C3?". If the user confirms such a question, then a new axiom of the application domain that does not follow from the ontology has been discovered, and it is added to the ontology. If the user rejects it, the user has to give a counterexample to this question, i.e., an individual that is an instance of C1, C2 and not C3. When all such questions (about some initially given classes) have been answered, the ontology is complete with respect to the application domain.