DC glow discharges in pure O2 in a Pyrex tube were studied to determine dissociation rate constant over a wide range of E/N and thereby to probe O2 dissociation cross section close to threshold. Electric field, E, was found from probe measurements while the gas density, N, from the gas temperature derived from the O2(b1g )  O2(X3g-) emission spectrum. O atom density (as well ratio O/N ratio) was measured by HR TALIF while O/N ratio was also determined by Ar actinometry. Time-resolved actinometry of partially-modulated discharges was used to probe the O loss rate. The O2 dissociation rate constant was determined as a function of E/N, and compared to calculations from different O2 cross section sets. This comparison allowed validation of a the self-consistent cross section set for O2.