Harbor oscillations can trap and amplify the wave energy and then cause damage to wharves, ships, and coastal buildings. In this study, the effect of harbor oscillations on Hambantota Port in Sri Lanka was investigated using the Simulating WAves till SHore (SWASH) numerical model. To gain insights into the impact of harbor oscillations on wave dynamics and sediment motion, the study examined three dimensionless parameters, namely amplification factor, Shields number, and Rouse number, to quantify the characteristics of the oscillating waves and describe the initiation of sediment motion and the modes of sediment transport. The study identifies four modes of oscillations in Hambantota Port, and SWASH model performed well in simulating the characteristics of these modes. It was found that long incident waves were more likely to amplify the oscillating wave amplitude within the harbor. In addition, high flow velocity, high Shields number, and low Rouse number were observed at the end of the thin dams and breakwaters at the entry of Hambantota Port for long incident waves, while for short incident waves, these values were observed in the near-shore area. The modal structures of oscillations, however, had little impact on flow velocity and sediment motion.