A theory is presented that describes the direct generation of electromagnetic O-mode emission via mildly energetic electron beams in a highly dense and warm plasma. In previous theories, direct generation of O-mode emission from mildly energetic electron beams (1--10 keV) was considered unlikely because the Cerenkov wave phase speeds are much greater that the electron beam velocity, making the waves and electrons nonresonant. However, it is demonstrated that when cyclotron harmonic and relativistic effects are considered, resonance between particles and waves can occur under the conditions that &ohgr; be very near &ohgr;p, and near (but not exactly at) m&OHgr;e,m being the cyclotron harmonic number. In this case, the calculated wave phase speeds can become very low (<2¿107 m/s), thereby establishing resonance with the electrons. Such conditions do exist in regions where interplanetary type II emission are generated, in the terrestrial foreshock region, and in planetary magnetospheres. ¿ 2001 American Geophysical Union