We represent LSDA+U calculations of the magnon frequencies of transition-metal monoxides (TMOs) using the frozen magnon approach. We systematically vary U to investigate the effect of the on-site Coulomb repulsion on various properties such as the gap, the magnetic moment, and the magnon energies. For all TMOs, a similar behavior of the gap and moment with respect to U is found: the gaps start to increase linear and saturate for large U. The moment is enhanced by ≈ 0.4μ_B towards the pure spin limit. Magnon energies are strongly suppressed on increasing the on-site Coulomb repulsion. The reason for this behavior is related to charg localization.