The solubility of O-2 was measured gravimetrically in 16 ionic liquids (ILs) containing piperidinium, pyrrolidinium, ammonium, tetrahydrothiophenium, methyltriazabicyclodecene, or guanidinium cations paired with bis(trifluoromethanesulfonyl)imide, bis(perfluoroethylsulfonyl)imide, or perfluoropropanoyl(trifluoromethylsulfonyl)imide anions. The temperature dependence of the solubility was determined for eight of the ILs. Henry law constants were determined from the solubility data as well as standard enthalpies and entropies of absorption for systems with temperature-dependent data. Knowledge of these thermodynamic properties is essential in guiding the structural design of ionic liquids for different applications involving O-2. The O-2 solubility did not follow any obvious trend with regard to IL structure or molar volume; instead, the dipole-quadrupole interactions are the primary driving forces for O-2 dissolution in ionic liquids, as demonstrated by negative standard enthalpies of absorption for all of the ILs. In addition, diffusion coefficients of O-2 in the ILs as a function of temperature (293, 313, and 333 K) and applied O-2 pressure (1, 3, and 5 MPa) were obtained by fitting the time-dependent absorption data to a widely used mass diffusion model. The diffusion coefficients of O-2 in ionic liquids at 293 K and 1 MPa roughly correlate inversely with the pure IL viscosity, likely due to the relatively low and similar O-2 solubilities under these conditions. As expected, O-2 diffusivities increase with increasing temperature. However, they also increase with increasing pressure, likely due to decreasing solution viscosity with increasing gas solubility.