  We measured the gas abundance profiles in a sample of 122 face-on spiral
galaxies observed by the CALIFA survey and included all spaxels whose line
emission was consistent with star formation. This type of analysis allowed us
to improve the statistics with respect to previous studies, and to properly
estimate the oxygen distribution across the entire disc to a distance of up to
3-4 disc effective radii (r$_e$). We confirm the results obtained from
classical HII region analysis. In addition to the general negative gradient, an
outer flattening can be observed in the oxygen abundance radial profile. An
inner drop is also found in some cases. There is a common abundance gradient
between 0.5 and 2.0 r$_e$ of $\alpha_{O/H} = -\,0.075\,\rm{dex}/r_e$ with a
scatter of $\sigma = 0.016\,\rm{dex}/r_e$ when normalising the distances to the
disc effective radius. By performing a set of Kolmogorov-Smirnov tests, we
determined that this slope is independent of other galaxy properties, such as
morphology, absolute magnitude, and the presence or absence of bars. In
particular, barred galaxies do not seem to display shallower gradients, as
predicted by numerical simulations. Interestingly, we find that most of the
galaxies in the sample with reliable oxygen abundance values beyond $\sim 2$
effective radii (57 galaxies) present a flattening of the abundance gradient in
these outer regions. This flattening is not associated with any morphological
feature, which suggests that it is a common property of disc galaxies. Finally,
we detect a drop or truncation of the abundance in the inner regions of 27
galaxies in the sample; this is only visible for the most massive galaxies.
