The resonance condition for maximum absorption of the energy carried by ocean waves incident to an oscillating wave energy converter (WEC) is one of the most well known results of the WEC control research. It was shown that the maximum occurs when the WEC velocity is in phase with the wave excitation force. The condition was obtained with the assumption that the WEC frees to oscillate with whatever amplitude was necessary, and that the power takeoff (PTO) system can deliver the unlimited force. In practical WEC implementations, this assumption does not hold. The purpose of this paper is twofold. First, to obtain the frequency response of the closed-loop WEC motion, and of the optimal control law in terms of the hydrodynamic coefficients with limited PTO force and limited motion amplitudes. Secondly, the obtained results are used to investigate the resonance condition of the WEC velocity with the incoming wave in the presence of constraints.