The oxygen (O2) compatibility of six polymers used in O2 service was evaluated after 48 h exposures at 121 °C (250 °F) to O2 pressures of 620 and 6200 kPa (90 and 900 psia). Three elastomers were tested: chloroprene rubber (C873-70), fluorocarbon rubber (Viton® A), and silicone rubber (MVQ type); and three thermoplastics were tested: polyhexamethylene adipamide (Zytel® 42), polytetrafluoroethylene (PTFE Teflon® 500A), and polychlorotrifluoroethylene (Neoflon® CTFE M400H). Post-aged changes in mass, dimensions, tensile strength, elongation at break, durometer hardness, and thermomechanical properties were evaluated. For selected materials, gaseous nitrogen- (GN2-) aged controls were used to ascertain the effect of thermal versus oxidative degradation. Finally, the effect of O2 aging on selected ignition (AIT) and combustion (ΔHc) properties was determined. As expected, aged polymers were less ignitable and combustible. Special attention was given to Neoflon CTFE; namely, the possible effect of percent crystallinity (quick- versus slow-quenched) on the aging, ignition, and combustion behavior was evaluated.