The generated power of Oscillating Water Columns (OWC) based on Wells turbines is limited due to the stalling behavior. Therefore, this paper presents the modeling of an OWC and a rotational speed control using a novel Fuzzy Gain Scheduled-Sliding Mode controller (FGS-SMC) for a stalling-free operation. The proposed SMC-rotational speed control will regulate the turbo-generator angular velocity to avoid the stalling behavior and increase the generated power. In an effort to reduce the fluctuations in the generated power, Fuzzy Logic Supervisors (FLS) were designed to adaptively schedule the switching gains of the SMC controller to reduce the chattering and improve its performance. A comparative study has been carried out between the FGS-SMC, the SMC and uncontrolled OWC using irregular waves and real measured waves. Results show the effectiveness of the proposed controls against the uncontrolled case and the superior performance of FGS-SMC over the SMC ensuring power generation improvement.