A model is presented for estimating oxygen solubility in water and solutions of inorganic electrolytes (I) as st function of oxygen pressure P-O2 (atm), temperature T (K), and I. It is based on a thermodynamic analysis for water, where the molal concentration c(aq) of oxygen follows an equation of the form c(aq) = Po(2)k and k is a T-dependent function (equilibrium constant) related to the chemical potential, entropy, and partial molar heat capacity of the gaseous oxygen (O-2)(g) and dissolved oxygen (O-2)(aq) species. In the presence of I, the oxygen solubility becomes (c(aq))(I) = phi c(aq), where phi is a modifying factor < 1 that is dependent on I and its molal concentration C-I. The decreasing molar heat capacity of (O-2)(aq) with rising T, which affects k, is discussed. The decrease in phi with increasing C-I is related in a general way to the decrease in partial molar volume of the water.