A lattice dynamical study has been carried out to investigate the Raman frequencies and the effect of pressure on the apical-oxygen (O A ) vibration (Raman Active) in the Hg O A Cu bond in HgBa 2 CuO 4 high-T c superconductor by using an unscreened rigid ion model. The calculated Raman frequencies are, in general, in fair agreement with experimental values. The calculated values of the O A A 1 g Raman mode agree reasonably well with the experimental Raman data up to 7.5 GPa and show that the O A A 1 g vibrational mode increases linearly as the pressure is increased, suggesting phonons to play a dominant role in this system.