In any health care system, the patient medical history is crucial to help the doctors for further patient diagnosis. History of the patient is collected mainly through face-to-face interaction when the patient visits the hospital. If the medical staff is not well experienced, it results in failure of collecting the patient history. So in most of the cases, effective patient risk analysis cannot be done. This paper proposes an ontology based system to collect the patient history and to assess the patient risk factors due to smoking history, alcohol history, erectile dysfunction history, and cardiovascular history. According to the patient history, a total score is calculated for each of the above factors. According to the score, the ontology performs the risk assessment on a patient profile and predicts the potential risks and complications of the patient. Ontology is among the most powerful tools to encode medical knowledge semantically.