The number density O2 (b1g ) molecules in the positive column of a dc discharge in pure O2 was determined by tow methods : high-resolution vacuum ultraviolet (VUV) absorption of the (4pπ1Σu b1g ).band at 131.3nm (using the Fourier Transform Spectrometer (FTS) at the DESIRS beamline at Synchrotron Soleil) and absolute intensity measurements of the integrated A-band (b1g ,v=0 X3g-,v=0) emission around 760nm. The 131.3nm absorption band was chosen because it is the only one that is well separated from O2 X and O2 a absorption bands. An analysis of the rotational structure of this heavily-perturbed band is presented as well as an ab initio calculation of the (previously-unknown) transition strength. The densities obtained by the two methods agree within 20%, confirming the accuracy of the transition strength calculation. The O2 b density increases with O2 pressure up 2 Torr (to about 1 % of the total gas density), then decreases at higher pressure. At low pressure the b density increases with discharge current, whereas opposite trend is observed at high pressure. These observations can be explained by O2 b quenching by O 3P atoms with a rate that increases with gas temperature.