Oscillating water column (OWC) is one of optional renewable energy device that been used for converting kinetic energy from waves energy into electrical energy. Suitable structural design must be measured and determine to make sure structure is capable to resist wave load. The scope of the research is to design of closed structure of OWC. A model has been set up in Ansys AQWA and Ansys Static Structural which has been undergone through several steps. The model has been run with selected significant wave frequency which is from article finding of the research. Hydrodynamic diffraction analysis has been carried out to identify maximum wave pressure acting on model. Maximum wave pressure be one of component to determine maximum equivalent stress (von Mises) and total deformation of OWC closed structure. There are 4 detail design structure models in this research. Furthermore, there are also 4 different position of maximum wave pressure acting on structure model. Parameter that involve in research including type of material, type of stiffener, number of stiffener and number of frame. Optimization process also carried out by selected suitable detail design structure model to minimize maximum equivalent stress (von Mises) and total deformation. Result shows the optimization process de-creased on structure up to 20–70% at different position for maximum equivalent stress (von Mises). Total deformation also decreased on structure up to 25–80% at different position.