  We study the stability of the results of 3-nu oscillation analysis of
atmospheric and reactor neutrino data under departures of the one--dominant
mass scale approximation. In order to do so we perform the analysis of
atmospheric and reactor neutrino data in terms of three--neutrino oscillations
where the effect of both mass differences is explicitly considered. We study
the allowed parameter space resulting from this analysis as a function of the
mass splitting hierarchy parameter alpha = Delta m^2/Delta M^2 which
parametrizes the departure from the one--dominant mass scale approximation. We
consider schemes with both direct and inverted mass ordering. Our results show
that in the analysis of atmospheric data the derived range of the largest mass
splitting, Delta M^2$, is stable while the allowed ranges of mixing angles
sin^2 theta_{23} and sin^2 theta_{13} are wider than those obtained in the
one--dominant mass scale approximation. Inclusion of the CHOOZ reactor data in
the analysis results into the reduction of the parameter space in particular
for the mixing angles. As a consequence the final allowed ranges of parameters
from the combined analysis are only slightly broader than when obtained in the
one--dominant mass scale approximation.
