Decades of research have contributed to a better understanding of the biomechanical basis of trauma brain injury (TBI) due to blunt impact. However, significantly less is known about the effects of blast overpressure (BOP) on the head biomechanics. In particular, the exact mechanism of how BOP causes TBI is not clear. In this paper, we investigate the mechanical coupling between the BOP and the head using a finite element model of the human head-combat helmet complex. To this end, fluid structure interaction simulations were performed to investigate the interaction of the BOP with the head-helmet complex. Our results showed that the BOP could be amplified within the gap separating the helmet and the head. This amplified pressure field could lead to increased load on the brain, consequently resulting in a higher likelihood for TBI. Our simulations also showed that the presence of helmet padding could reduce the pressure field around the head, which may result in a decreased possibility of TBI. Our findings could be useful towards efforts to improve the design of current combat helmets at protection against BOP-induced TBI.