The optical emission spectra of microwave oxygen plasmas under various gas pressures have been investigated. At the gas pressure of 0.01 Torr the plasma-generated particles are mainly excited atomic oxygen and ionic molecular oxygen species. As the gas pressure is increased to a value higher than 0.2 Torr, the plasma contains excited atomic oxygen with emission peaks at 777 and 844 nm and a new emission peak at 759 nm. When the gas pressure reaches 2.9 Torr, only a single emission peak at 759 nm appears. To the best of our knowledge, the optical emission spectra peak at 759 nm has not been reported before. This peak is thought to be due to the presence of excited molecular oxygen species. Al low gas pressures, the concentrations of atomic oxygen and ionic molecular oxygen increase linearly with increasing input power to the plasma, while there is no such input power dependence for excited molecular oxygen. SiO2 films have also been fabricated by means of oxidation of silicon wafers in microwave oxygen plasmas at two gas pressures, 0.01 and 2.2 Torr. It is found that the SiO2 films grown at 0.01 and 2.2 Torr have properties similar to those of thermally grown oxide, and that the films grown at 0.01 Torr have a lower value of interface trap density (approximate to 9X10(-10) eV(-1) cm(-2)) and a smaller leakage current. Atomic oxygen species are found to be the main species to contribute to the oxidation of silicon in microwave oxygen plasmas.