Oxygen atoms are a key reactive species in many plasma applications. Its absolute density is often determined by two-photon laser-induced fluorescence (TALIF) by comparing the signal amplitude with that obtained from a known density of Xe atoms, as proposed by Niemi et al.[1] However, this method has limited accuracy due to uncertainty in the relative excitation cross-sections and other detector calibration difficulties. Our recent measurements in a DC glow discharge yielded unphysically high values, motivating a search for a new method. Absorption spectroscopy is the most direct and unambiguous method for determining absolute densities. However, under most practical situations the O atoms resonance lines at 130nm are heavily saturated (OD of 100 or more) and cannot be used. We measured the vacuum ultraviolet absorption spectrum of O2 discharges at high (106) resolution using the Fourier-Transform Spectrometer[2] on the DESIRS beamline at Synchrotron Soleil, and succeeded in observing the forbidden 3P25S2 transition at 135.56nm. This transition is not saturated, allowing the O atom density to be determined with high accuracy and the TALIF method to be tested. Another key parameter in plasmas is the gas translational temperature, and the thermal accommodation coefficient at reactor walls. We have developed a method to determine the temperature of O atoms from the Doppler width of the TALIF transition, using a home-built narrow bandwidth (0.01cm-1 @ 226nm) pulsed laser system [3], and will use this to determine the temperature profiles in an O2 ICP, and hence the surface thermal accommodation coefficients. Work supported by LABEX Plas@par / ANR-11-IDEX-0004-02.