Electrical discharges in oxygen gas are widespread in nature and occur in many plasma applications such as surface treatment and plasma medicine. They are also an ideal archetype for the understanding of molecular plasmas, showing the effects of dissociation/surface recombination, electron attachment, high densities of molecular and atomic metastable states, vibrational excitation and gas heating. Nevertheless, many uncertainties remain about the cross-section and reaction sets, as well as the surface processes, all essential for reliable modelling. We are making comprehensive measurements absolute densities and kinetics of the principal transient species in a well-characterised and reproducible system, a DC positive column in pure O2. This provides a uniform plasma column with constant reduced field over a wide range of gas pressure and electron density (pressure 0.2-10 Torr, current 5-40 mA), ideal for model validation or improvement. A range of powerful diagnostic techniques are employed, including vacuum ultraviolet (120-200nm) absorption spectroscopy, cavity ringdown spectroscopy (CRDS), time resolved optical emission spectroscopy and high resolution two-photon laser-induced fluorescence (HR-TALIF) The Fourier-Transform branch of the unique DESIRS beamline at synchrotron Soleil was used to record absorption spectra with a resolution of 106, giving complete high-resolution spectra of O2 in the X, a and b states. The absorption cross-sections of O2 X and a are known, allowing absolute densities to be determined. Using a combination of ab-initio calculations and absolute emission spectrometry, we have derived the first absolute absorption cross-sections of O2 b, and therefore made the first the absolute measurements of density in this state. Time-resolved measurements of O2 X, a and b during both full and partial modulation of the discharge current, using the monochromatic branch of the VUV beamline, allowing their creation and loss kinetics (in the gas phase and on the tube surface) to be probed. The electron-impact dissociation coefficient is poorly known at low reduced electric field, but it can be determined from measurements of the absolute density of oxygen atoms, combined with loss rate measurements (from time-resolved emission). However, the absolute density is hard to measure accurately. Unfortunately, the resonance lines at 130nm are entirely saturated and therefore useless. Therefore we have developed cavity-ringdown spectroscopy at 630nm (using the forbidden 1D ← 3P transition), allowing the density to be determined with an accuracy better than 10%. Additionally the O- negative ion density can be determined from the photodetachment continuum in this spectral region.