This paper studies the nonlinear wave loads on a stationary platform that consists of four cylindrical Oscillating Water Column (OWC) Wave Energy Converters (WECs). The nonlinear wave action on the quad-OWC platform was simulated using a 2nd-order time-domain Higher-Order Boundary Element Method (HOBEM). A carefully instrumented experiment was carried out to validate HOBEM simulations. A circular orifice was introduced to model the nonlinear Power Take-Off (PTO) system on the ceiling of the OWC chamber. Subsequently, the sensitivities of the nonlinear wave loads to the incident wave direction and array spacing were investigated. The optimum angle of wave incidence was found to be π/4, i.e., when the quad-OWC platform is diagonally aligned with the incoming waves. A 2nd-order sloshing resonant mode was found inside each chamber of the OWC array, which leads to an increase of the wave loads. A good selection of the column and row spacing contributes to the reduced wave forces on both the front and rear OWC devices. The directions of the wave loads on the front device were analyzed. The rotational-angle peaks of the wave force and moment were always observed at high frequencies.