Operation support system (OSS) has been investigated to support the operator decision making in abnormal plant condition. The usefulness of OSS depends on the efficiency of the utilized operation decision method. This paper proposes an enhanced operation decision support system (ODSS), as an important function in OSS, using plant design information. At the plant design stage, the potential factors of dangerous situations are clarified and the causal relationships between causes and effects are investigated as a result of the risk assessment practices. Safety design and operating procedures of abnormal situations are examined based on such knowledge, which can also be useful to decide the appropriate operation at the plant operation stage. This means that plant design rationales can be reflected into the operation decision making. Accordingly, the system can show the reason of selecting the next operation. Prototype system of ODSS is developed based on the proposed operation decision method and applied to HDS process as a case study.