  We study the conditions for the existence of neutrino oscillations in the
field-theoretical approach which combines neutrino production and detection
processes in a single Feynman graph. The ``oscillating neutrino'' is
represented by an inner line of this graph where, due to the macroscopic
distance L between source and detector, the neutrino propagators for neutrinos
with definite mass are replaced by the projection operators unto the neutrino
states on mass shell. We use as a concrete model reaction the neutrino source
and detector as given in the LSND experiment and we carefully take into account
the finite lifetime of the stopped muons which provide the anti-muon neutrino
beam. We show that the field-theoretical approach provides a solid method to
locate all possible conditions and allows to separate unambiguously their
different origins. Some of these conditions are independent of L whereas others
state that coherence is lost when L exceeds a certain ``coherence length''.
Also it turns out that, at least in the concrete situation considered here, the
concept of neutrino wave packets is not supported by the field-theoretical
approach for realistic experimental conditions, i.e., the neutrino energy
spread is incoherent in origin.
