Undoped n-type MgZnO films were deposited on c-plane sapphire substrates by molecular-beam epitaxy and subsequently annealed in O-2 at different pressures. After annealing at 3.03 x 10(5) Pa, oxygen content in the annealed films show increases and the films transform into p-type conduction. However, the decreases of oxygen content and the increases of electron concentration were obtained while the films annealed at 1.01 x 10(5) Pa or 2.05 x 10(-3) Pa. The changes in intensity of the emission peak located at 2.270 eV are similar to the changes of the oxygen content in the films annealed at different pressures. According to the defect levels and the relationship between photoluminescence spectra and annealing condition, it was suggested that this emission peak was related to interstitial oxygen (O-i). The obtained p-type conduction is attributed to that the O-i acceptor can compensate oxygen vacancy and interstitial zinc donor.