  We present a homogeneous analysis of the oxygen abundance in five H II
regions and eight planetary nebulae (PNe) located at distances lower than 2 kpc
and with available spectra of high quality. We find that both the collisionally
excited lines and recombination lines imply that the PNe are overabundant in
oxygen by about 0.2 dex. An explanation that reconciles the oxygen abundances
derived with collisionally excited lines for H II regions and PNe with the
values found for B-stars, the Sun, and the diffuse ISM requires the presence in
H II regions of an organic refractory dust component that is not present in
PNe. This dust component has already been invoked to explain the depletion of
oxygen in molecular clouds and in the diffuse interstellar medium.
