A two-dimensional time-domain, fully nonlinear numerical wave tank (NWT) technique based on potential theory, the mixed Eulerian-Lagrangian (MEL) approach, and the boundary element method was developed and applied to a land-based oscillating water column (OWC) system with compressed air inside the chamber. The nonlinear free-surface inside the chamber was specially treated to represent both the viscous effect of the water column motion and the pneumatic pressure of the time-varying airflow velocity in the chamber. The developed NWT was verified through comparison with viscous-flow-based numerical and experimental results by Liu et al. (2008) with and without air ducts. The NWT simulations correlated well with experimental values with tuned viscous damping coefficient for a variety of wave conditions. The fully nonlinear simulations were also performed with irregular wave input, so that the developed numerical tool can be practically used for the optimal design of land-based OWCs.