Co/CoO nanocomposite thin films have been deposited by RF sputtering with different oxygen partial pressures in the working atmosphere. It has been found that the magnetic properties (exchange bias (EB), coercivity and remanence) of the films are strongly dependent on the oxygen partial pressure. Film deposited with an O-2 pressure of 0.75 x 10(-5) Torr shows a relatively high coercivity and high remanence ratio. Enhanced asymmetry of magnetization reversal appears in the film deposited with 1.5 x 10(-5) Torr O-2 partial pressure. When the O-2 partial pressure is increased to 3.0 x 10(-5) Torr, a strong reduction in EB and coercivity have been obtained. The investigation of the microstructure using high-resolution transmission electron microscope reveals that the change in EB, remanence and coercivity is probably related to the competition between dipolar interaction and exchange coupling. Magnetoresistance and extraordinary Hall effect (EHE) of the Co/CoO nanocomposite films is also investigated in dependence on oxygen partial pressure. The film deposited under 2.3 x 10(-5) Torr of oxygen exhibits strongly enhanced asymmetric magnetization reversal and unusually high EHE with a high coercivity. (C) 2004 Elsevier B.V. All rights reserved.