A potential flow model is derived for a large flap-type oscillating wave energy converter in the open ocean. Application of Green’s integral theorem in the fluid domain yields a hypersingular integral equation for the jump in potential across the flap. The solution is found via a series expansion in terms of the Chebyshev polynomials of the second kind and even order. Several relationships are then derived between the hydrodynamic parameters of the system. Comparison is made between the behaviour of the converter in the open ocean and in a channel. The degree of accuracy of wave tank experiments aiming at reproducing the performance of the device in the open ocean is quantified. A parametric analysis of the system is then undertaken. In particular, it is shown that increasing the flap width has the beneficial effect of broadening the bandwidth of the capture factor curve. This phenomenon can be exploited in random seas to achieve high levels of efficiency.