A small microwave ion source has been constructed to generate low‐energy (5–10 eV) oxygen beams. The source is fabricated from a quartz tube and is enclosed by a microwave cavity. The source is operated without an external extraction voltage. Positive ions effuse from the source with energies equal to the plasma potential. The source has been operated in a cw mode producing an atomic oxygen ion fluence >1×1014 cm−2 s−1with energies as low as 5.5 eV. The ‘‘self‐extracted’’ ion beam can be used to simulate the oxygen environment encountered in low earth orbits.