This innovative work investigated two models where the muonium-antimuonium oscillation process was mediated by massive Majorana neutrinos and sneutrinos. First, we modified the Standard Model only by the inclusion of singlet right-handed neutrinos and allowing for general renormalizable interactions producing neutrino masses and mixing. The see-saw mechanism was employed to explain the smallness of the observed neutrino masses. A lower bound on the righthanded neutrino mass was constructed using the experimental limits set by the nonobservation of the muonium-antimuonium oscillation...