Loop Calculations and OZI Rule

The role of two step mechanisms in the OZI rule violation in NNbar annihilation at low energies is reviewed. The full calculation of two-meson doorway mechanisms shows that the off-mass-shell contributions are important. Moreover, for the (phi pi0) and (phi rho) annihilation channels three-meson doorway contributions are non-negligible and improve agreement with the data. A significant enhancement of phi production is found in the resonant rescattering mechanisms p pbar -> pi+ pi- K Kbar -> pi+ pi- phi and p pbar -> eta K Kbar -> eta phi.