In this study, advanced numerical CFD tools based on OpenFOAM are developed to generate extreme waves for the investigation of wave-in-deck loads on fixed offshore structures. The numerical method employs a Navier-Stokes solver with the free-surface captured by the volume of fluid (VOF) approach. The advantages of numerical simulation through CFD are: 1) full scale simulation of nonlinear phenomena; 2) potentially more accurate prediction as viscous and inertia forces are included whereas a model test based on Froude scaling can only scale the inertia forces; and 3) detailed insight into the flow and resulting loads. The numerical simulations for long-crested extreme waves capture the evolution of impact loads and free surface during the interaction with the platform deck. CFD provides more accurate results than analytical tools and the flow physics can be studied in detail, providing further insight that can lead to improvements in the analytical tools. This study also provides a benchmark set of problems for wave loading on platform decks.