The complicated geometries of hydrated anion clusters are investigated using density functional calculations. The structures of the first solvation shell of O-(H2O)(n), O-2(-)(H2O)(n), and O-3(-)(H2O)(n) (n = 0-4) were calculated at the B3LYP/aug-cc-pVDZ level, focusing on the hydrogen bond distances between the anion and the water, the change of the water O-H bond lengths, and the H-O-H angles for these species systematically. These parameters are related to the binding energy between the anion and the water, and the binding energy tends to decrease as the number of water molecules and the size of anions increase. Three kinds of interactions in these clusters were found as follows: (i) the strong anion-water interaction; (ii) the water-water interaction; and (iii) the weak anion-water interaction. The strong anion-water interactions are seen in all of the species. However, the water-water interactions are seen only in O-2(-)(H2O)(2) with C-1 symmetry, O-2(-)(H2O)(3), and O-2(-)(H2O)(4), and the weak anion-water interactions are seen only in O-3(-)(H2O)(n). These geometrical parameters are important to understand the structures of hydrated anions. Furthermore, the standard thermodynamic functions for X-(H2O)(n-1) + H2O --> X-(H2O)(n), (X = O, O-2, and O-3) were calculated to investigate their stability in the atmosphere. -DeltaHdegrees and -DeltaGdegrees decrease as the number of water molecules and the size of the anion increase. These thermodynamic functions are useful to understand the anion hydration.