This paper presents numerical investigations with experimental comparisons of random wave induced loads on the external walls of a large scale Oscillating Water Column (OWC) device. The objective is to investigate the capability of a numerical model that uses the Reynolds Averaged Navier–Stokes (RANS) equations to accurately reproduce extreme wave loads on an OWC. Both incompressible and compressible approximations for the RANS equations are studied in this work with the aim of investigating the limitations of both approaches. After a first characterisation of the random wave propagation processes at the numerical flume, wave loads are analysed. An OWC power take-off (PTO) and in-chamber air compressibility effects are studied using regular waves. Next, an OWC facing irregular waves is studied and compared with experimental data, putting special emphasis on the extreme loads together with the stochastic nature of random waves. The simulation of extreme wave loads on an OWC is validated and the necessity of a compressible model for correct reproduction of an OWC in-chamber flow for certain operating conditions of the PTO is pointed out from the simulations.