  We consider the pumping of the $63.2 \mu$m fine structure line of neutral OI
in the high--redshift intergalactic medium (IGM), in analogy with the
Wouthuysen--Field effect for the 21cm line of cosmic HI. We show that the soft
UV background at $\sim 1300$\AA can affect the population levels, and if a
significant fraction of the IGM volume is filled with ``fossil HII regions''
containing neutral OI, then this can produce a non--negligible spectral
distortion in the cosmic microwave background (CMB). OI from redshift $z$ is
seen in emission at $(1+z)63.2\mu$m, and between $7<z<10$ produces a mean
spectral distortion of the CMB with a $y$--parameter of $y=(10^{-9} -
3\times10^{-8}) (Z/10^{-3}{\rm Z_{\odot}}) (I_{UV})$, where $Z$ is the mean
metallicity of the IGM and $I_{UV}$ is the UV background at 1300\AA in units of
$10^{-20}$ erg/s/Hz/cm$^2$/sr. Because O is in charge exchange equilibrium with
H, a measurement of this signature can trace the metallicity at the end of the
dark ages, prior to the completion of cosmic reionization and is complementary
to cosmological 21cm studies. While future CMB experiments, such as Planck
could constrain the metallicity to the $10^{-2} Z_{\odot}$ level, specifically
designed experiments could potentially achieve a detection. Fluctuations of the
distortion on small angular scale may also be detectable.
