This study aims proposing a novel geometry of the onshore Oscillating Water Column (OWC) device to increase its efficiency. A comparison between standard OWC's and the new proposal ones, in which the exterior front wall is inclined in opposite direction from that of the incident wave, is carried out. Simulations of 2D flows, with incompressible water and compressible air, are performed by means of the FLUENT® software, which is based on Reynolds-Averaged Navier-Stokes equations. The k-ϵ turbulence model and the Volume of Fluid method are employed. Analyses of the hydrodynamic and aerodynamic behavior, run-up/down on the exterior front wall, sloshing inside the chamber and the energy balance of OWC devices equipped with the Wells turbine and with different wall slopes (40° − 90° from horizontal) are carried out for incident regular waves with periods from 6 to 12 s and height of 1.5 m. The new proposal OWC's provide higher extracted energy than standard ones. The best performances are reached by wall slope of external front wall of 40° and internal wall slopes of 40° and 52°, although the more efficient solution depends on the sea state characteristics of the region where the OWC will be installed.