The origin of large-amplitude hydromagnetic waves in the Venus magnetosheath down-stream of the quasi-parallel bow shock is investigated by means of numerical simulation. It is shown that the most likely source of these waves is the bow shock itself, rather than an instability involving the solar wind and oxygen ions of planetary origin. Pickup of O+ ions by these waves is also examined and shown to be in agreement with previous test particle calculations. The effect of mass loading on the structure of the shock is also discussed.