We discuss in detail solar neutrino oscillations with δm^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 δm^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.

17 pages (RevTeX) + 8 figures (PostScript). Final version, to appear in Phys. Rev. D