  In this work, we study the lepton number violating W boson and top quark
decays via intermediate on-shell Majorana neutrinos Nj into three charged
leptons and a light neutrino. We discuss the neutrino oscillation effects
present in the decay due to the small mass gap between the heavy neutrino
states. We focus on a scenario that contains at least two heavy Majorana
neutrinos in the mass range 2-80 GeV. The results indicate that the modulation
of the branching ratios as a function of the distance between the vertices may
be detected in a future experiment such as High-Luminosity Large Hadron
Collider. As a secondary result, the CP-violating phases could be explored.
