  We apply on-shell amplitude techniques to the study of neutrino oscillations
in vacuum, focussing on processes involving W bosons. We start by determining
the 3-point amplitude involving one neutrino, one charged lepton and one W
boson, highlighting all the allowed kinematic structures. The result we obtain
contains terms generated at all orders in an expansion in the cutoff scale of
the theory, and we explicitly determine the lower dimensional operators behind
the generation of the different structures. We then use this amplitude to
construct the 4-point amplitude in which neutrinos are exchanged in the
s-channel, giving rise to oscillations. We also study in detail how flavor
enters in the amplitudes, and how the PMNS matrix emerges from the on-shell
perspective.
