  We present Herschel-PACS spectroscopy of four main-sequence star-forming
galaxies at z~1.5. We detect [OI]63micron line emission in BzK-21000 at
z=1.5213, and measure a line luminosity, L([OI]63micron) = (3.9+/-0.7)x1.E+9
Lsun. Our PDR modelling of the interstellar medium in BzK-21000 suggests a UV
radiation field strength, G~320 G0, and gas density, n~1800 cm-3, consistent
with previous LVG modelling of the molecular CO line excitation. The other
three targets in our sample are individually undetected in these data, and we
perform a spectral stacking analysis which yields a detection of their average
emission and an [OI]63micron line luminosity, L([OI]63micron) =(1.1+/-0.2)x1E+9
Lsun. We find that the implied luminosity ratio, L([OI]63micron)/L(IR), of the
undetected BzK-selected star-forming galaxies broadly agrees with that of
low-redshift star-forming galaxies, while BzK-21000 has a similar ratio to that
of a dusty star-forming galaxy at z~6. The high [OI]63micron line luminosities
observed in BzK-21000 and the $z \sim 1 -3$ dusty and submm luminous
star-forming galaxies may be associated with extended reservoirs of low
density, cool neutral gas.
