Full dimensional (15 df) quantum Monte Carlo calculations of vibrational energies of H5O2+ are reported using the OSS3(p) potential energy surface of Ojamae et al. [Ojamae, L.; Shavitt, L; Singer, S. J. J. Chem. Phys. 1998, 109, 5547]. The protonated water dimer, H5O2+, is an important case for vibrational study for the purpose of understanding the potential energy surface of the smallest aqueous proton-transfer system. Vibrational calculations are quite difficult for this system because of its high dimensionality and strong coupling between anharmonic low-frequency coordinates, among them motion of the central proton, which is associated with the most intense vibrational modes. In our study, the correlation function quantum Monte Carlo (CFQMC) method is employed for a full-dimensional treatment of the system. The development of an improved trial wave function gave a better starting point for understanding the excited states and was a crucial step that enabled us to obtain a set of converged energy eigenvalues using the CFQMC method. Several methods of extrapolating energy eigenvalues from the CFQMC data were evaluated. Transition moments were calculated to help identification of the excited states and allowed us to obtain exact information about most of the fundamental frequencies of the H5O2+ ion.