Fixed Oscillating Water Column (OWC) and Wavestar Wave Energy Converter (WEC) are two standard converters with different strategies but the same goal, extracting energy from waves. Fixed-OWC is a coastal system that uses pneumatic pressure with a turbine, but Wavestar uses a power take-off (PTO) damping system to extract power and operate in both shore and offshore areas. The innovation of the present study is using these two systems simultaneously in a shared platform, in the sense that fixed-OWC is used as the base structure for Wavestar convertor. For this purpose, three main designs with variable distances between the Wavestars' floating object and the fixed-OWCs' front wall, WEC-17.5, WEC-17.8, and WEC-18.1, were performed for four different wavelengths using the numerical solution. The solver is OverInterDymFoam from the standard library of the OpenFOAM solvers, considering both dynamic mesh (Overset mesh technique) and wave generation schemes. Although the idle combination is extracting power from Wavestar besides no efficiency reduction for fixed-OWC, a decrease in efficiency for some cases is inevitable. An overall assessment of the proposed combined system for 12 different case studies reveals that there is an efficiency reduction in some cases, near 38% for the worst case. Still, the superiority of this method is efficiency increment up to 13% for system design points "the efficient span in which the convertor works." Finally, a structural analysis was performed to calculate the exerted force on the front wall of fixed-OWC in the new combined platform.