We describe an atomic oxygen source based on a D.C. plasma discharge, compatible with cristal growth in a Molecular Beam Epitaxy(M.B.E.) system. The physical characteristics of the oxygen cell are presented. The efficiency of the cell has been proved by direct deposition of CuO at high temperature(500°C). Moreover, we used successfully this cell for direct epitaxial growth of high temperature superconductors, with an ambient pressure as low as 2 10-5 Torr.