We report the first anion photoelectron spectroscopic study of nickel monoxide (NiO), nickel dioxide (ONiO), and nickel-O-2 complex, Ni(O-2). The adiabatic electron affinity (EA) of NiO is measured to be 1.46 (2) eV. Five low-lying electronic excited states (A (3) Pi, a (1) Delta, B (3) Phi, b (1) Sigma(+), c (1) Pi) are observed for NiO at 0.43 (4), 0.94 (4), 1.24 (3), 1.80 (10), and 2.38 (10) eV above the ground state, respectively. Two isomers are observed for NiO2, i.e., the linear ONiO dioxide and the Ni(O-2) complex. The dioxide has a high EA of 3.05 (1) eV while the Ni(O-2) complex has a rather low EA of 0.82 (3) eV. Two low-lying excited states are observed for ONiO at 0.40 (2) and 0.77 (3) eV above the ground state, respectively. The vibrational frequency of the nu(1) mode of the ground state ONiO (X (3) Sigma(g)(-)) is measured to be 750 (30) cm(-1). The excited states of the Ni(O-2) complex give broad photodetachment features starting at about 1.1 eV above the ground state. Information about the electronic structures of the nickel oxide species and chemical bonding between Ni and O and O-2 is obtained and discussed.