  We present the largest and most homogeneous catalog of HII regions and
associations compiled so far. The catalog comprises more than 7000 ionized
regions, extracted from 306 galaxies observed by the CALIFA survey. We describe
the procedures used to detect, select, and analyse the spectroscopic properties
of these ionized regions. In the current study we focus on the characterization
of the radial gradient of the oxygen abundance in the ionized gas, based on the
study of the deprojected distribution of HII regions. We found that all
galaxies without clear evidence of an interaction present a common gradient in
the oxygen abundance, with a characteristic slope of alpha = -0.1 dex/re
between 0.3 and 2 disk effective radii, and a scatter compatible with random
fluctuations around this value, when the gradient is normalized to the disk
effective radius. The slope is independent of morphology, incidence of bars,
absolute magnitude or mass. Only those galaxies with evidence of interactions
and/or clear merging systems present a significant shallower gradient,
consistent with previous results. The majority of the 94 galaxies with H ii
regions detected beyond 2 disk effective radii present a flattening in the
oxygen abundance. The flattening is statistically significant. We cannot
provide with a conclusive answer regarding the origin of this flattening.
However, our results indicate that its origin is most probably related to the
secular evolution of galaxies. Finally, we find a drop/truncation of the oxygen
abundance in the inner regions for 26 of the galaxies. All of them are
non-interacting, mostly unbarred, Sb/Sbc galaxies. This feature is associated
with a central star-forming ring, which suggests that both features are
produced by radial gas flows induced by resonance processes.
