Many considerable efforts and advances have been made worldwide concerning the wave power absorption by floating wave energy devices based on the Oscillating Water Column (OWC) principle in the open sea. However, much less attention has been paid to the influence of the moorings on the wave power absorbed by an OWC. The aim of the present paper is to examine the effect of different mooring systems on the efficiency of an OWC device. The method of 'matched' eigenfunction expansions for the velocity potential in the fluid domain surrounding the device is used to provide the necessary solution to the diffraction problem (body fixed in waves, atmospheric pressure in the chamber); the pressure radiation problem which results from an oscillating pressure head acting on the inner free surface of the OWC and the motion radiation problem resulting from the forced oscillation of the device in its six degrees of freedom, with atmospheric pressure in the chamber. Furthermore, considering the coupled dynamical model for the present analysis, i.e., floater and mooring system, the characteristics of several examined mooring systems are combined with the hydrodynamics of the OWC and the exciting wave loads to set up the coupled motion equations. Numerical results obtained through the developed solution in the frequency domain are presented, highlighting the motion modes where the mooring line characteristics are critical as well as their effect on the OWC’s wave power absorption.