PIROG8 with a newly developed receiver for observing the 425GHz O 
line was launched from the CNES balloon-base in southern France on Sept.
25, 1997 giving an 8 hr flight at float altitude. The main target was the
giant molecular cloud associated with NGC7538 and its O 425GHz line
was observed for about two hours. In order to connect our O
observations to the CO abundance, the C 
O(1-0) line was observed
with the Onsala 20m telescope and from the 
arcmin map we
derived the PIROG beam weighted spectrum. From these data it was concluded
that the O abundance is less than 4% of the CO abundance which is the
lowest upper limit yet determined for a Galactic cloud. Current chemistry
calculations predict far more (by an order of magnitude) molecular oxygen
and there is at present no convincing explanation why the abundance is much
lower than expected.