  A substantial fraction of oxygen in diffuse clouds is unaccounted for by
observations and is postulated to be in an unknown refractory form, referred to
as unidentified depleted oxygen (UDO), which, depending on the local gas
density, may contribute up to 50% of the total oxygen content. Previous
Infrared Space Observatory (ISO) observations suggest that a significant
fraction of oxygen in even denser, translucent clouds may be in atomic form. We
have analyzed velocity-resolved archival SOFIA observations of the 63 $\mu$m
fine-structure [O I] transition toward the high-mass star-forming region Sgr
B2(M) in the Central Molecular Zone. The foreground spiral-arm clouds as well
as the extended Sgr B2 envelope between the Sun and the background dust
continuum source produce multiple [O i] absorption components, spectrally
separated in velocity space. The gas-phase atomic oxygen column density in
foreground clouds toward Sgr B2 is well correlated with the total hydrogen
column density, with an average atomic oxygen abundance of $(2.51 \pm 0.69)
\times 10^{-4}$ with respect to hydrogen nuclei. This value is in good
agreement with the earlier ISO measurements on the same line of sight, and is
about 35% lower than the total interstellar medium oxygen abundance in the
low-density warm gas, as measured in the UV. We find no evidence that a
significant fraction of the oxygen on the line of sight toward Sagittarius B2
is in the form of UDO.
