Add to ORKGWe present a formulation, based on quantum field theory, for neutrino oscillation phenomenology that can be applied to nonrelativistic and relativistic energies for neutrinos. The formulation is constructed as the time evolution of a lepton family number density operator. We also introduce a Gaussian momentum distribution for the initial state that incorporates wave packet-like decoherence effects. At time $t=0$, we assign either a Dirac or Majorana mass to the neutrino. Then, the time evolution of the lepton family number density operator becomes dependent on the mass and new features appear. We show in the nonrelativistic regime, by taking the expectation value of the density operator, the absolute mass hierarchy and the type of neutrino ...