  We present the first detection of neutral atomic oxygen (3P_1-3P_2 fine
structure line at ~63um) toward the Horsehead photodissociation region (PDR).
The cloud has been mapped with the Spitzer Space Telescope at far-IR (FIR)
wavelengths using MIPS in the spectral energy distribution (SED) mode. The
[OI]63um line peaks at the illuminated edge of the cloud at AV~0.1-0.5 (inward
the gas becomes too cold and outward the gas density drops). The luminosity
carried by the [OI]63um line represents a significant fraction of the total FIR
dust luminosity (I_63/I_FIR~4x10^-3). We analyze the dust continuum emission
and the nonlocal OI excitation and radiative transfer in detail. The
observations are reproduced with a gas density of n_H~10^4 cm^-3 and gas and
dust temperatures of T_k~100 K and T_d~30 K. We conclude that the determination
of the OI 3P_J level populations and emergent line intensities at such ``low''
densities is a complex non-LTE problem. FIR radiative pumping, [OI]63um
subthermal emission, [OI]145um suprathermal and even maser emission can occur
and decrease the resulting [OI]63/145 intensity ratio. The Herschel Space
Observatory, observing from ~55 to 672um, will allow us to exploit the
diagnostic power of FIR fine structure lines with unprecedented resolution and
sensitivity.
