In this paper we propose a circle of care (COC) ontology that specifies concepts and relations necessary to capture a patient's circle of care and allows one to make inferences about who is in a patient's circle of care. The ontology can improve current access control systems in making decisions regarding access events in real time and help identify past cases of illegitimate access through the access log. We validate the ontology by augmenting the Fast Healthcare Interoperability Resources (FHIR) data model with our COC ontology and presenting an example of access logs from FHIR that is extended to answer circle of care queries.