The cross second virial coefficients and dilute gas binary diffusion coefficients of water (H2O) in oxygen (O2) and air were predicted with high accuracy for temperatures up to 2000 K using statistical thermodynamics and the kinetic theory of polyatomic gases. For this purpose, we developed a new H2O-O2 intermolecular potential energy surface, which is based on high-level quantum-chemical ab initio calculations and represented analytically as a site-site potential function. To determine the cross second virial coefficient and the dilute gas binary diffusion coefficient of the (H2O + air) system, we combined the calculated values for the (H2O + O2) system with available first-principles results from the literature for the systems (H2O + N2), (H2O + Ar), and (H2O + CO2). The calculated property values for both systems agree satisfactorily with experimental data and form the sole basis of new reference correlations, which we provide here as well.