The development of devices for extracting wave energy from the ocean is largely supported by numerical models, as they allow the simulation of different configurations without the large costs of tank testing. From the different available options, time-domain models offer a very good combination between accuracy, flexibility and computational time. They allow the incorporation of non-linearities from power take-off systems, mooring lines, sophisticated control techniques and other relevant hydrodynamic effects. In this paper, we present a time-domain model to simulate the dynamics and power performance of a slack-moored Spar-buoy OWC (Oscillating Water Column) wave energy converter. The model considers linear hydrodynamics, mean drift forces, viscous drag effects and air compressibility inside the OWC chamber. The mooring system is simulated using a quasi-static approach. The floating structure is defined as a rigid body with six degrees of freedom, whereas the OWC free surface is assumed flat. The converter motion and power extraction from regular and irregular wave simulations are compared with experimental results from small-scale model tests in a wave channel. Numerical results show good agreement with experimental data except when parametric resonance is observed and near the channel cut-off frequencies.