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 concerning the cross-section and reaction sets, as well as surface processes, all necessary for reliable modelling. We are making comprehensive measurements of transient species absolute densities and kinetics 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. We employ a range of powerful diagnostic techniques, including vacuum ultraviolet absorption spectroscopy (120-200nm) using the unique DESIRS beamline at synchrotron Soleil. The Fourier-Transform branch 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. However, some differences in the spectra are seen due to our superior resolution. 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 the absolute density of this state. The absolute density of oxygen atoms is a critical but hard to measure parameter. It is needed to calculate the electron-impact dissociation cross-section. Unfortunately, the resonance lines at 130nm are entirely saturated and therefore useless. However, we able to demonstrate absorption via the forbidden 5S203P2 transition at 135.56nm, allowing relative [O] measurements by other techniques (actinometry, TALIF) to be calibrated. We also performed 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. However, interpretation of this data is complicated by both gas temperature variations and spectral overlap, causing both gas density modulation and possibly spectral shape changes. An improved, dual-beam setup is under development to address these issues. Measurements were also performed in a low-pressure (10-50 mTorr) ICP discharge, giving the absolute O2 X, a and b densities as a function of pressure and RF power, allowing recent speculation on the role of metastable states to be tested.