O-vacancy formation energies in the bulk and at the low index surfaces of typical cathode materials, including layered LiMO2 (M Co, Ni, Mn) and LiNi1/3Co1/3Mn1/3O2 (NCM-333), spinel LiMn2O4, and Li-rich Li2MnO3 for Li-ion batteries, are studied from density functional theory calculations. The thermodynamic stabilities of the oxygen vacancies in these materials are discussed. Except for the spinel type LiMn2O4, all considered materials are more likely to form oxygen vacancies after delithiation, as the vacancy formation energies are lower in their delithiated states. The delithiated Li2MnO3 is very unstable both in its bulk and at its surfaces, as the vacancy formation energies are negatively very large. For layered LiMO2, the (003) surfaces are more stable than the (104) surfaces for all M cases, but Mn is able to stabilize O atoms in the (104) surface of the NCM-333. Temperature can have a significant influence to O-vacancy formation, as formation energies may change from positive to negative when the temperature increases from 300 K to 800 K, for some cases. The oxygen partial pressure does not have obvious influence to the O-vacancy formation, but increasing pressure can also be practical treatment to suppress the formation of O-vacancies under high temperatures.