  We present Herschel-PACS spectroscopy of the [OI]63um far-infrared cooling
line from a sample of six unlensed and spectroscopically-confirmed
870um-selected submillimetre (submm) galaxies (SMGs) at 1.1<z<1.6 from the
LABOCA Extended Chandra Deep Field South (ECDFS) Submm Survey (LESS). This is
the first survey of [OI]63um, one of the main photodissociation region (PDR)
cooling lines, in SMGs. New high-resolution ALMA interferometric 870um
continuum imaging confirms that these six Herschel-targeted SMG counterparts
are bona fide sources of submm emission. We detect [OI]63um in two SMGs with a
SNR >3, tentatively detect [OI]63um in one SMG, and constrain the line flux for
the non-detections. We also exploit the combination of submm continuum
photometry from 250-870um and our new PACS continuum measurements to constrain
the far-infrared (FIR) luminosity, L_FIR, in these SMGs to < 30%. We find that
SMGs do not show a deficit in their [OI]63um-to-far-infrared continuum
luminosity ratios (with ratios ranging from ~0.5-1.5%), similar to what was
seen previously for the [CII]158um-to-FIR ratios in SMGs. These observed ratios
are about an order of magnitude higher than what is seen typically for local
ultra luminous infrared galaxies (ULIRGs), which adds to the growing body of
evidence that SMGs are not simply `scaled up' versions of local ULIRGs. Rather,
the PDR line-to-L_FIR ratios suggest that the star formation modes of SMGs are
likely more akin to that of local normal (lower-luminosity) star-forming
galaxies, with the bulk of the star formation occurring in extended regions,
galaxy-scale (~kpc) in size. These observations represent the first step
towards a census of the major PDR cooling lines in typical SMGs that will be
attainable with ALMA, enabling detailed modelling to probe the global
properties of the star formation and the evolutionary status of SMGs.
