  We discuss in detail solar neutrino oscillations with \delta m^2/E in the
range [10^-10,10^-7] eV^2/MeV. In this range, which interpolates smoothly
between the so-called ``just-so'' and ``Mikheyev-Smirnov-Wolfenstein''
oscillation regimes, neutrino flavor transitions are increasingly affected by
matter effects as \delta m^2/E increases. As a consequence, the usual vacuum
approximation has to be improved through the matter-induced corrections,
leading to a ``quasi-vacuum'' oscillation regime. We perform accurate numerical
calculations of such corrections, using both the true solar density profile and
its exponential approximation. Matter effects are shown to be somewhat
overestimated in the latter case. We also discuss the role of Earth crossing
and of energy smearing. Prescriptions are given to implement the leading
corrections in the quasi-vacuum oscillation range. Finally, the results are
applied to a global analysis of solar nu data in a three-flavor framework.
