In the present work, we have studied the performance of an open compression chamber with compressed air and driven by an oscillating water column. Recognizing the existence of a free-surface for the water column, the interface position between the trapped-air and water volume -together with the motion of the column-, is described by a non-linear energy equation that reflects the main dynamic characteristics. The above governing equation is posed in dimensionless form and solved by conventional numerical methods. In addition, a theoretical approximation of the first order in ε to predict the resonant frequency of the oscillatory system is derived to complete the analysis. The numerical results of the above governing equation serve us to estimate the dimensionless work done by the oscillating water column as a function of three dimensionless parameters: a characteristic Froude number, and two equivalent quasi-geometric parameters, β and ε, defined below. The predictions show that the influence of the geometry and the involved physical parameters exert a great influence on work generation into the air-chamber.