Thermochemistry of radicals is not as extensively tabulated as that of stable molecular species, even when group additivity schemes are applied. When these radicals contain oxygen or nitrogen atoms, the availability of radical groups is even more limited. Many oxygen- and nitrogen-containing radicals and molecules are present in the atmosphere, and thermochemistry is a valuable component of the development of atmospheric models with predictive capabilities. This paper presents quantum chemical calculations using G3//B3LYP that have been performed to obtain heats of formation, entropies, and heat capacities as a function of the temperature of radicals and molecules from which group additivity values were obtained. Isodesmic and homodesmotic reactions were used to obtain improved estimates of the heats of formation. Thermodynamic property estimates were corrected to account for internal rotations. A total of 323 molecules were studied from which a total of 122 different groups, 21 gauche and cis corrections, and 5 secondary corrections were regressed.