  In a scenario with two almost mass degenerate heavy sterile Majorana
neutrinos with a mass $\sim 1$ GeV, we present the semileptonic decay widths of
heavy charged pseudoscalars such as $B$ mesons, either lepton-number-violating
($B^{\pm} \to \mu^{\pm} e^{\pm} \pi^{\mp}$), or lepton-number-conserving
($B^{\pm} \to \mu^{\pm} e^{\mp} \pi^{\pm}$), mediated by such on-shell
neutrinos. It had been explained in the literature that such decays may be
eventually detected, and that they can present even detectable CP violation
effects. In this work we point out that, in addition, such decays may present
detectable effects of heavy neutrino oscillation, allowing us to extract the
oscillation length and thus the heavy neutrino mass difference $\Delta M_N$, as
well as a CP-violating Majorana phase.
