The electronic and geometrical structures of oxygen-rich neutral and negatively charged FeO5, FeO6, FeO7, FeO8, FeO9, FeO10, FeO11, and FeO12 clusters were obtained using density functional theory with generalized gradient approximation. With the exception of FeO11 and FeO12, all clusters are found to possess a large number of isomers composed of oxo, peroxo, superoxo, and ozonide fragments that are closely spaced in total energy, especially for n = 7 and 8. The preferable structures of FeO12 are composed of superoxo groups with different orientations. All the neutral species possess rather large electron affinities, which range from 3.24 eV (FeO8) to 3.95 eV (FeO5). Although all of the lowest energy states were found to possess positive vibrational frequencies and thus are geometrically stable, the states are thermodynamically unstable against dissociation to FeO4 + (n - 4)/2 O-2 for n = 6, 8, 10, and 12 and FeO5 + (n - 5)/2 O-2 for n = 7, 9, and 11. In particular, the decay of FeO12 is exothermic by 34 kcal/mol.