  This paper assesses the implications of a recent discovery (Jenkins 2009)
that atomic oxygen is being depleted from diffuse interstellar gas at a rate
that cannot be accounted for by its presence in silicate and metallic oxide
particles. To place this discovery in context, the uptake of elemental O into
dust is considered over a wide range of environments, from the tenuous
intercloud gas and diffuse clouds sampled by the depletion observations to
dense clouds where ice mantles and gaseous CO become important reservoirs of O.
The distribution of O in these contrasting regions is quantified in terms of a
common parameter, the mean number density of hydrogen. At the interface between
diffuse and dense phases (just before the onset of ice mantle growth) as much
as 160 ppm of the O abundance is unaccounted for. If this reservoir of depleted
oxygen persists to higher densities it has implications for the oxygen budget
in molecular clouds, where a shortfall of the same order is observed. Of
various potential carriers, the most plausible appears to be a form of
O-bearing carbonaceous matter similar to the organics found in cometary
particles returned by the Stardust mission. The "organic refractory" model for
interstellar dust is re-examined in the light of these findings, and it is
concluded that further observations and laboratory work are needed to determine
whether this class of material is present in quantities sufficient to account
for a significant fraction of the unidentified depleted oxygen.
