An offshore platform has several modules that contain much of the equipment needed for oil and gas production, and these are placed on the limited space of the topside. Furthermore, the equipment layout should leave sufficient space in between to ensure operability, maintainability, and safety. Thus, the design problem to arrange the topside of an offshore platform can be difficult to solve due to the number of modules and equipment placed on the topside. This study proposes a method to arrange the offshore topside based on an expert system and multistage optimization in order to obtain the optimal arrangement that addresses various considerations and satisfies the given requirements. The proposed method consists of four components. First, an expert system is proposed to systematically computerize experts' knowledge and experience and to evaluate the feasibility of alternatives for the arrangement of the offshore topside. Second, a multistage optimization method is proposed to yield a better arrangement design by formulating the arrangement design problem as an optimization problem with two stages. Third, an arrangement template model (ATM) was proposed to store the arrangement data of the offshore topside. Fourth, the user interface was developed to run the expert system and for optimization. A prototype program was then developed to solve an floating, production, storage, and offloading (FPSO) topside problem in order to evaluate the applicability of the proposed method. The results showed that the proposed method can be used to obtain the optimal arrangement of an offshore topside.