We report a photoelectron spectroscopy (PES) study on a series of fullerene oxides, C60Ox- (x = 1-3). The PES spectra reveal one isomer for C60O-, two isomers for C60O2-, and multiple isomers for C60O3-. Compared to C-60, the electronic structures Of C60Ox are only slightly perturbed, resulting in similar anion photoelectron spectra. The electron affinity Of C60Ox was observed to increase only marginally with the number of oxygen atoms, x, from 2.683 eV for C-60, to 2.745 eV for C60O, and 2.785 eV/2.820 eV for C60O2 (two isomers). We also carried out theoretical calculations, which confirmed the observed isomers and showed that all the fullerene oxides are in the form of epoxide. The PES and theoretical calculations, as well as molecular orbital analysis, indicate that addition of oxygen atoms to the C-60 cage only modifies the local carbon network and leave the rest of the fullerene cage largely intact geometrically and electronically.