The present work is focused on the assessment of how OpenFOAM performs when applied to non-linear wave interactions with offshore structures for ranges of wave conditions. New modules have been further extended to advance the wave generation and wave absorbing capabilities of the code. The numerical results for wave interactions with a vertical surface piercing cylinder have been compared with physical experiments performed at Danish Hydraulic Institute (DHI). Comparisons between the numerical results and the measured data for three regular waves and four focused wave groups, have indicated that OpenFOAM is very capable of accurate modelling of nonlinear wave interaction with offshore structures, with up to 4th order harmonic correctly captured. Moreover, by using the crest-trough phase-based separation method, we can reproduce harmonic structure in the wave loading on the structure and free surface elevations.