Atomic oxygen, an abundant upper atmospheric species, is an important target in collisions involving fast ions and neutrals precipitating into the Earth's thermosphere. To facilitate the laboratory study of these collisions and to measure their differential cross sections, an appropriate atomic oxygen target has been developed. Oxygen atoms for the target are produced by electron-impact dissociation of O\sb2 in a discharge created in a microwave cavity resonant at 2.54 GHz. Thirty percent of the added molecular oxygen at 50 mTorr is dissociated in the discharge. The atomic and molecular oxygen mixture flows through a phosphoric acid coated pyrex tube to a fluorocarbon coated target cell. The dissociation fraction in the target cell is determined by titration with NO\sb2 and by analysis with a time-of-flight mass spectrometer.