Updated analyses of several singlet Rydberg states of O 2 via spectra involving excitation from the metastable a 1 Δ g and states are presented. The high-quality FT-VUV spectra available from the DESIRS beamline at the SOLEIL synchrotron gives significantly improved spectra compared to previous work. The Rydberg states analysed include v = 0-4, 3pσ 1 Π u v = 0-2, 3pπ 1 Δ u v = 0-2, v = 0-1, 4pσ 1 Π u v = 0 and 4pπ 1 Δ u v = 0. This is complemented by high-quality ab initio calculations on the and 1 Δ u Rydberg states to determine the transition moments providing the first quantitative cross-sections for Rydberg - transitions. These are validated against the experimental data. The results suggest the most promising candidate for determining number density is likely to be the 1-0 band of the transition at 131.3 nm.