  We propose a Quantum Field Theory (QFT) approach to neutrino oscillations in
vacuum. The neutrino emission and detection are identified with the
charged-current vertices of a single second-order Feynman diagram for the
underlying process, enclosing neutrino propagation between these two points.
The key point of our approach is the definition of the space-time setup typical
for neutrino oscillation experiments, implying macroscopically large but finite
volumes of the source and detector separated by a sufficiently large distance
$L$. We derive an $L$-dependent master formula for the charged lepton
production rate, which provides the QFT basis for the analysis of neutrino
oscillations. Our formula depends on the underlying process and is not
reducible to the conventional approach resorting to the concept of neutrino
oscillation probability, which originates from non-relativistic quantum
mechanics (QM). We demonstrate that for some particular choice of the
underlying process our QFT formula approximately coincides with the
conventional one under some assumptions.
