  We study the conditions for neutrino oscillations in a field theoretical
approach by taking into account that only the neutrino production and detection
processes, which are localized in space around the coordinates $\vec{x}_P$ and
$\vec{x}_D$, respectively, can be manipulated. In this sense the neutrinos
whose oscillations are investigated appear as virtual lines connecting
production with detection in the total Feynman graph and all neutrino fields or
states to be found in the discussion are mass eigenfields or eigenstates. We
perform a thorough examination of the integral over the spatial components of
the inner neutrino momentum and show that in the asymptotic limit $L=|\vec{x}_D
- \vec{x}_P| \rightarrow \infty$ the virtual neutrinos become ``real'' and
under certain conditions the usual picture of neutrino oscillations emerges
without ambiguities.
