Clinical Decision Support System (CDSS) is a health information system which is designed to help the physicians in decision-making process. However, it remains a challenge to successfully provide and implement CDSS in clinical practice. One of the issues is regarding the limited provisions of causal reasoning and supporting evidences for the presented outcomes or decisions in existing CDSS. These are necessary for showing the relevance and reliability of the outcomes, and helping the physicians in making proper decisions. This study has designed an ontology-based CDSS model that integrates the domain medical knowledge with causal reasoning, decision support, and supporting evidences. The CDSS model is developed for the case study of Methadone Maintenance Therapy (MMT). From the evaluation by the ontology metrics, the results show that our model is feasible and comparable to other ontology models. As a result, our CDSS model able to provide full support to the CDSS requirements particularly in the provision of causal reasoning compared to other CDSS models, and these signify the contributions of this study.